The very assumption of it being impossible to coexist with other entities capable of reason is without merit. The Necron, Tyrannids, and Orks are good examples of why 'only war' exists in the setting is that they cannot be reasoned with at all.
Both Orks and Necrons can be reasoned with and often work with/for various other races. It's just rarely a long lived arrangement since both races have their own agendas and motives which are usually most readily pursued at the expense of others.
The Tyranids can't be reasoned with, but that's not because they aren't sapient (Norn Queens are fully sapient hyperintelligent beings) but rather that they see everything that isn't a higher stage Tyranid lifeform the way we view cattle and have no interest in communing with them. It'd be like talking to a beefburger.
The unwillingness of the Eldar to work with other races is actually quite sensible when you understand their perspective. A consistent part of their depiction throughout the history of 40k is that they find humans (and other races) as inherently
disgusting. We breathe wrong, talk wrong, move wrong, look wrong, build wrong, everything about humans is wrong to them to lesser or greater extent. Add that a lot of human colonies are built on old Eldar worlds and they view humans in much the same way as we'd view the lovechild of a Lovecraftian monster and a chimpanzee with Down's Syndrome that had been beaten with an ugly stick and just happened to be living in a hovel made of human bones. Of course they're just as wrong in every way to human perceptions as well, so the feeling is mutual.
As for the Tau, they possess a world view that is inherently dangerous. Their naivety has killed multiple worlds before now. They've made deals with races like the Dark Eldar that saw entire planets of their people dragged into Commorragh to be tortured and enslaved, they've interfered in Chaos rituals that saw entire worlds burned in sacrifice to the Chaos Gods. The fact they enslave humans in gulags and are only held together by gentle mind control is largely irrelevant compared to the fact that these are the sort of people who would allow races like the Yuu'Vath or the Slaugh to walk in and become citizens. Hell, they'd permit chaos worship in their empire on the grounds that they don't believe in gods and that petty superstitions are better being phased out than stamped out.
It's best to think of the various Xenos races as being eldritch monsters (especially the Eldar), they're alien in thought, form and deed. No one would try to negotiate with the Slaugh or the Rak'Gol, nor should they, and the Eldar and Tau are no less alien and inimical to human life.
Could somebody explain me the hype with those sexy models? I mean, did the genestealers models got off production for a while? Or rectoned or something else?
Genestealers have had models for ages, the Genestealer Cultists haven't had any for something like 2 decades even though Tyranid players have been begging for them for about as long. The fact that they have mini-rules (that are actually decent) as well is just icing on the cake to the people who've been holding out for them.
Genestealers are one of the iconic Tyranid lifeforms, they've been around since before the 'nids themselves, so they have a pretty strong fanbase.