The same way the US would end the world if Russia started building bases in Mexico. In fact, this exact scenario already happened during the Cuban missile crisis. Anybody who keeps peddling the notion that Russia should just accept NATO encroachment is deeply intellectually dishonest.
It’s obvious to everyone with even a minimally functioning brain that Russia isn’t just going to pack up and go home after nearly three years of war. Especially given that Russia is very obviously winning the war at this point. NATO has a choice to accept reality that they lost or to start a nuclear holocaust. It’s pretty clear that there are plenty of imbeciles living in NATO countries who would prefer the latter.
Meanwhile, this whole narrative of appeasement is the height of bullshit. Everybody appeased the US and NATO when they invaded Iraq, Libya, Afghanistan, Syria, and countless other countries. The US currently occupies a larger percentage of Syria than Russia is of Ukraine. The appeasement just means that NATO has to accept that another country is doing what NATO does regularly.