What makes you think the worst parts won’t still get through or be allowed through…like troll farms or hacking groups?
Well if they cut themselves off, then why wouldn’t western nations just finish the job and block all Russian IPs from connecting? Make them travel outside of Russia to set up propaganda machines. Sure, it won’t have much of an effect, but it’ll annoy them, and that’s something.
They’d just do the same thing anyone does when the want to obscure their home IP. Just use VPNs, route though sympathetic counties, or other ways to hide who you are. It’d just be a constant game of whack-a-mole. Why do you think we can’t just block people who hack our infrastructure or companies can’t so easily block data thieves? If all it took was blocking a couple addresses or a range it would be easy.