Ukraine could win, but nobody wants to help with that.

Properly funded and supplied with a full arsenal (not limited to short-range or outdated gear), they would win in a few years (the time for full training on the gear). The reason why they can't/wont win otherwise is that they just don't have enough offensive weapons or air superiority enough to really take the war to the Russians. They've only ever made limited strikes into any Russian targets in Russia proper. The US/NATO has held back because of Putin's threats of escalation.

Ukraine can't/won't win alone, but that doesn't mean they shouldn't be helped. It's not a sunk cost yet, but the better bet to me would be for them to return to their original borders as it would give them a better chance to eventually re-pay all of the aid. The average US taxpayer gets nothing if Ukraine loses but has a chance of getting some of the aid repaid if they win.