“As this would restore the situation in Ukraine to the “status quo ante” that existed before Putin ordered the invasion”
That was the reality on the ground before the first shot was fired...excepting the political formalities of such.
No it wasn't. Ukraine had the inherent right of any sovereign nation to join other nations in collective self-defense. Putin is demanding it surrender that right.
And even on the ground, the pro-Russia separatists only controlled portions of Donbas and Luhansk. Russia's proposal has them more than doubling that territory.