I also don't see Russia backing down without some type of agreement that cedes territory or other significant concessions to them, which sets up a sort of moral hazzard. Then, a few years from now Russia does this sort of thing again. It would feel like the sort of appeasement policy used with Germany leading up to WWII.
I don't see a good way around that possibility that doesn't risk a huge escalation. Actually I may see one very imperfect option: Cede Crimea and maybe other concessions to Russia in exchange for and agreement of neutrality backed by allowing a very large build up if Ukrainian military power, and a guarantee of support from the West, to significantly deter Russia from doing this again. At the same time that specific and severe sanctions beyond what we've seen already will be rapidly implemented if Russia tries this with any other country.
Even this is problematic, and probably unrealistic that Russia would agree to it. And so it just comes down to how long Ukraine can hold out, or how strong of an insurgency they can mount for the long term & what price Russia is willing to pay (eventually they cut their losses in Afghanistan). But Russia probably decides that if they can't have Ukraine then then they'll destroy every city and as much infrastructure as possible on their way out (hints of that already)
It is truly hard for me to see any positive outcome here that is as likely as any of the many bad possible outcomes. Maybe if Putin's purges of political leadership cause then to launch a coup in self defense? Waging war among that chaos would be hard, and the West could negotiate with new leadership about getting out of Ukraine in exchange for lifting some of the sanctions.