Tau stealth tech is still detectable, and only found on a small number of their units. That and being invisible doesn't help when the enemy has enough men to literally drown you in blood and corpses if they want to and enough artillery to turn night into day. The Tau just get ground down by the sledgehammer that is the guard while their leadership gets taken out in precision strikes by Space Marines. Most fights the Tau have 'won' is just because the Imperium had to go deal with something that's actually a threat, like Chaos, Orks or Tyranids. It's important to remember that thought Tau tech is slightly better than the Imperium's, it's still behind the Dark Age of Technology relics the Imperium possess and the Tau empire is only about a hundred worlds in size compared to the millions of worlds in the Imperium.
The Imperium has destroyed a few Tau worlds in the 2nd and 3rd expansions of the Tau empire, but it usually doesn't bother. Tau worlds can be reclaimed just fine unlike those of Chaos or Orks, so exterminatus is usually only done to them to deny the Tau ground and destroy any military assets they have present. As for genociding the Eldar Craftworlds... I think the Imperium killed one, and lost most a segmentum fleet to do so. It's one of the hardest tasks in the Galaxy and since Eldar are usually peaceful enough to ignore it's not a common thing. Especially since the other Craftworlds will destroy a hundred humans worlds in revenge.
I will say that this is only partially accurate. Most
wars the Tau have won are because the Imperium had to go fuck off and fight other stuff.
Important thing to remember is that the Imperium is large enough that if it could focus on any one faction it could probably wipe it out, save maybe 'Nids and Chaos. The whole teetering back and forth on the edge of collapse and 'you only won because we had to fight someone else' is a fair chunk of the point. Tau tech is fairly better than the Imperium's, it's not Eldar or anywhere near it, of course, but Pulse Rifles are a step or five up from lasguns (wouldn't actually matter if Imperium was well-organized, don't get me wrong, numbers/experience advantage is big). Real advantage is their organization, efficiency, and willingness to innovate, really. AX-10 Tigersharks are a
very cheap counter to Titans, relative to the value of their targets. No, not perfect, need support, etc., but so does their newest 'actual' Titan (On the one hand, yay Tau Artillery finally. On the other hand...blugh...at least there's a fairly decent fluff reason for it that I won't go into here), like all things that can really kill Titans.
The description of the way the fights go is an...acceptable description, I suppose, of the way the Damocles Crusade went? I mean, just having stealth tech doesn't mean shit, I agree, it's not the end-all-be-all 'advantage' for the Tau in this mismatched fight. But Taros Campaign shows a pretty good example of the way Tau like to fight. And how it goes when they can't fight that way. Really, Imperials and Tau are tailor-made for each other (and if no one can see allegories to Great Crusade Humanity then I am disappoint in all of you, and so would the Emperor. Been wanting to detail that for a while.); Guard like to take and hold ground, and could give a rip about the casualties they take in the process; Tau are perfectly fine with letting you have the ground, they just want to slaughter you. The Tau also have precision strike units, and they also strike at leadership when they can. Just like the Space Marines, they use them for cutting supply lines and attacking key points, rather than just 'kill leaders do it go'.
Tau tech is getting better all the time (plus how many Dark Age relics can the Imperium spare?). And they can produce more of it. Of course, Tyranids mean they'll never get a chance, and if no Tyranids, then Imperium would have the energy to deal with them and make sure they didn't become more of an issue. But hypothetically, Tau could very well become a threat. Or get ripped apart by Chaos insurrections/Tau of Iron/'Hey Look Administering large swathes of space is hard' disease. Depends how you look at it. They are growing at more or less an exponential rate, after all, last I read. With all the other shit the Imperium has to deal with, once they got to about a thousand worlds they could very well spell Serious Difficulties, depending on tech level at the time. We'll never find out, of course, because this is 40k.
What I originally came to talk about was my wondering about what would happen if the Imperium started
winning. Like, sweeping reforms to get administratum properly efficient and organized, Warp calms down for who knows reason (Actual Reason: Slaanesh killed the Dark Eldar in a murder-orgy when they were a few seconds late in the torture rites; she's not temporarily contented, and this angers Khorne. Also they're all having fun crucifying Abaddon for fucking up one too many times in the eyes of his fickle Dark Lords; aka they got distracted), and yes I know Chaos would never calm down and blah in canon that's not the point. They find a cache of real nice STCs, and trickle-down effect...Imperium is on the brink of collapse. In a way, this means they are also on the brink of stability. If things stop going wrong all the time, for once (aka Chaos stops dicking around in things), and a couple of things go the Imperium's way, like the big Ork WAAAGH and the Tyranid Hive Fleet actually exhausting each other enough that the Imperium can step in and clean up if it has the manpower, then they can start pushing back the tide. Maybe they find a way to deal with Necrons more effectively, or it turns out the rest of the Tomb Worlds had some errors in the millenia they've laid undisturbed. Tyranids turn out to
not be the harbingers of Doom, DOOM, and MORE DOOM OH EMPEROR THE HORROR, and are actually a desperate Hive Mind after millenia without sustenance trying to get big enough to get to the next galaxy. Still big, tho. Just not THE SIZE OF THE GALAXY big. Orks just require liberal application of Orbital Bombardment and Heavy Flamers, Eldar will stop dicking around once they don't foresee bullshit fucking with their Craftworlds (yes I understand them not dicking around for the sake of dicking around is the most unbelievable part of this idea stick with me here tho). Imperium starts pushing back shit. Really helps when entire regiments don't fuck off into chaos all the damn time, and you can afford to synchronize commands a bit better. Mebbe Marneus Calgar comes awake enough to say 'stop being lone wolf dickheads, all Space Marines', before finally dying, redemption achieved. Honestly, only a few of these are really needed, only real vital one is Chaos deciding to take a break for a bit.
And I don't mean Emps getting up or dying and becoming Starkin or anything, or Leman Russ popping in to help, I just mean the ball starts rolling the other way. Tau get obliterated just as soon as the men can be spared for it. Orks get crushed and burned out of their nests, kept contained in other places, proper Machine God hygiene rituals followed. Necrons start being destroyed once sufficient men/Cyclonic torpedoes are thrown at them (The Flayed One were satisfied and went home, you see). Guard Regiments can start being used properly, in
truly overwhelming numbers. Nids get fought off, always at great cost but not always as great as you'd think. Imperium gets back to status as Big Boys In The Playground, With More Than Enough Sticks To Hit All Of You.
My question is what happens after that? How does it start to fall apart from within, even without Chaos, once it's basic driving motivator of 'fight for our existence as a species' is no longer quite accurate? Xenos are dealt with as much as they can be. Chaos worlds that we can destroy are being destroyed, the rest we're staying away from instead of losing Chapters for no reason until we can afford to spare them. Only generic traitors are left.
Empires fall once they get comfy. So how would the pieces crumble, from the inside (saying they get lazy and then Chaos wakes up or xenos start breaking shit defeats the point of this), once the Imperium could afford some decadence, do ou think?