So in your own description, if international support ends, Ukraine will be forced to surrender without a negotiated peace. On that basis, it is either "keep supporting them" or "Ukraine surrenders". Again, that's the situation as you have just described it.

Considering the total failure of both Minsk agreements, the fact that the pre-2014 borders were already based on a treaty with Russia that included security guarantees for Ukraine, and the fact that Russia has no right to anything out of this war, I don't expect Ukraine to really have a lot of faith in any negotiation in which they don't hold an extremely strong hand. The Russian government has demonstrated with Crimea that even if it takes something, it will not be satisfied there, it will be only be emboldened to try to take more. Following that, I say we should give them that strong hand.