MiGs alone aren't much of a weapon. Presumably the MiGs would come with air-to-air missiles, but how many? Also they need landing fields and fuel, and both would become more urgent targets for the Russians. In contrast, surface-to-air missile systems are much more mobile than landing fields and fuel/ammo storage. Finally, how much damage are Russian aircraft doing compared to Russian artillery? Would the MiGs be used for ground attack? If so, would they strictly target Russian forces within Ukraine and never leave Ukrainian air space? Finally, how many Ukrainian pilots trained on MiG-29s are without MiG-29s at the moment? Really finally, Polish uses Latin letters, Ukraine uses their own form of Cyrillic; would Ukrainian pilots be able to use Polish MiGs as-is?