If arming Ukraine does not substantially impact Ukraine’s ability to fight, how does it prolong the war?

This is not exactly what I said. I didn't say that it didn't impact Ukraine's ability to fight, I said it doesn't change the outcome of the war.

Of course, arming Ukraine adds difficulty for Russia, but it only at most delay Russia getting what they want since because of the way the war is going and the west's inability to outproduce Russia, Russia has time on their side. Russia can largely afford to just wait until western weapon supply to Ukraine can't keep up with theirs anymore, which is exactly what they have been doing since their retreat from the siege of Kiev in 2022, that's why the front line has barely moved since then, Russia know they are in a position where time will do most of the work for them.