Try it and Kiev is Russian in 2 months.
I disagree the West simply won't allow this. Europe and America alike have put up defenses across the region since the Russians started taking lands from the Ukraine. I do think that your bottom line notion is correct though - The Ukraine can not beat Russia in a military 1 on 1 battle.
I think that if Estonia, Lithuania and Latvia did not enter NATO in time, they suffered the same fate as Ukraine.