The only reason that the area of Ukraine that Russia is currently occupying is not being expanded is that despite expending thousands of lives and tonnes of equipment Poo Tins forces are incapable of taking more. Its absolutely nothing to do with voluntarily maintaining some "buffer zone", if Poo Tin was given half a chance he would roll right through Ukraine then choose his next target. That is why it is important for Europe to cut him off at the knees now so there is no chance that the mongrel can ever progress his barbaric plan.
