Again, the tau can afford to be nice because they don't have the same size an empire. You cannot expect a policy that works well on a small scale to work equally well on a grander scale. Hundreds of real life examples.
Should the imperium take quite a few lessons from the way tau do things? Heck yes. Should they wisen up and realise they really should ally (not join or subjugate to, ally) with this particular xenos to better combat the truly vile xenos? Yes. But subjugating to another species, of which you know next to nothing? Sounds like a bad idea. Who's to say the ethereal caste won't decide to start mass purges/sterilisation once they were in control? They don't have to do any such thing right now, if for the sole reason they aren't powerful enough for it (they are so utterly small on galactic scale, they can't afford but to take any ally or advantage they can get). But you have no guarantees.
Humans should stay in control of humanity, as long as that is reasonably possible.
For the record, I'm getting the feeling you might be underestimating the insidious power of Chaos a bit. Sure, better living conditions and such would go a long way to stemming cultist activity, but there will still be cults popping up every day everywhere. The imperium has a harsh police state and an entire organisation dedicated to detecting and purging chaos. And yet they often fail. What would the tau, or anyone else, even really do to combat that without using the same methods? After all, these massive hives are already here, so the issue is real.
Finally, sure the imperium has a bloated population, needs a lot of able hands. Getting rid of their fear of technology would help alleviate that somewhat, but still. They need to keep producing that many men, weapons, materiel, because they keep the forces of chaos and a lot of the aliens at bay.
Who holds the Cadian gate? Who fought back the three major tyranid incursions? Who keeps a lot of the orks down? Humanity, via the Imperium.