But wouldn't turning this into Putin's Afghanistan cause casualties to skyrocket? I can also see Putin getting frustrated and using tactical Nukes if this drags on. I also don't see any deterrent against him using Nukes in Ukraine as it wouldn't result in MAD.
Depends how you perceive "Putin's Afghanistan", I guess. As a failed protracted war or literally a decade long resistance? I don't think even the biggest optimistic would say that the Russian economy will last in this form of isolation for another decade, or that Putin will last that long either. And Ukraine is receiving way more help than Afghanistan was.
Also the longer this drags on, the more aggressive Putin is getting, but also the bigger the damage to the Russian economy and the more active the West gets at sending help. Would you have said a week ago that NATO would be sending anti-tank, anti-air missiles and even fighter jets to Ukraine?