the only apparent reason to prevent Ukraine trying to sue for peace is

Ukrainian nationalism.

Zelensky was suspected being a secret Russian puppet by them, they won't take it easy if he really becomes one (in their eyes). There won't be a peace, just a civil war, and way worse than the one in Ireland (that was already worse then the Irish war of independence that preceded it). Or the complete chaos like in Chechnya 1996-1999 (radicals making trouble after a peace treaty again), before the Russian tanks came back. It's not Georgia 2008, it's more like Georgia in the early 1990s.