> If they don't retreat from those regions while it's still an unknown, it will just give Russia pretext to invade entire Ukraine.

According to Ukraine, Russia invented the pretexts it has already cited for the “need” to intervene in those two regions, so from Ukraine's perspective, avoiding given Russia a “genuine” (given Russia’s nakedly illegal claims) pretext has zero marginal cost, since the absence of a “genuine” (even given Russia’s preexisting propaganda position) pretext has no evident impact on Russia’s willingness to act.

Given that, Ukraine has little rational reason to consider providing a pretext under Russia’s propaganda framework to be a significant factor.