Russia is not going to accept a 10 or 20 year insurgency on its doorstep just because UK/US would like it to go that way. This is not a laboratory war on the other side of the world for them. Moscow to Kiev is ~500 miles, about the same as Washington DC to Atlanta. Washington DC to Cuba is like 1100 miles and missiles in Cuba were completely unacceptable in the 60s when missiles were a fraction as nasty as they are now.

At some point, Russia is going to put their foot down and nuke the military forces in Ukraine, or at least the ones that aren't hiding in cities.

Then what, hotshots? Are you going to escalate to the full exchange over thug Biden's pocket money? Destroy Europe's economy and permanently lose all of Eurasia over Ukraine?