I said that without western support Ukraine military demise is inventible (emphasize on military). Otherwise, you seem to underestimate how Ukraine war effort dependent on outside support, for example how long doing you think that Ukraine anti air can hope to last without expansive ammo, early warning capabilities from NATO, fuel communication, etc.
No, I don't. Of course, centralized air defense will rather quickly stop being a thing (but note that existing stocks of manpads will be still MILES better than what Syrian Rebels had, and mobile short-range systems like BUK won't be hunted down instantly and existing stocks will last for some time)
But if the army losses a branch, it doesn't mean it collapses. Army collapses when there is either death to the chain of command or mass desertion.
As I said, the nature of war will change. And it will be a losing war that will gradually transition to guerilla but emphasis is on gradually. That if Russia can stomach all that war attrition in a years-long war.
You are missing my point: Syrian rebels waged conventional warfare with controlled areas having far less than what Ukraine has in terms of weapons, ammo, industrial capacity, trained manpower, territories, and political unity.
Also, as I said, I think no support at all is plainly impossible. Poland will help and the closer Russian troops will be to their borders, the more intense the help will be.