There's no evidence that Russia is going to lose steam economically or on the battlefield any time soon. Continuing to fight a losing war will only make any final deal between the US, Russia and Ukraine worse for the latter. There's a reason the 2022 treaty that was proposed looks unrealistic today, and whatever deal they make now will be much better than when they finally run out of men in the Ukrainian army.

With the situation as it stands, negotiating is the best way out if you actually care about Ukraine. If you just want to weaken Russia then sure, fight to the last ukrainian.