We haven't just discovered these demands. These were his demands before the war started. Presuming these are his true demands and he doesn't create more or change his mind, what's so tough about accepting these demands?

The provinces in question are largely Russian speaking provinces who prefer Russia. Crimea is lost and there is no hope of retrieving it under any circumstances. What does Ukraine really lose by accepting the demands?

The reality on the ground is the only reality that matters. The Russian army is all over Ukraine and while they are very destructive, they will only get more destructive the longer the war lasts. If he wins, he'll just impose his will anyway, and most military planners figure he will eventually win.

And if he wins, it will be worse. He will have a free hand to punish anyone who defied him. He will be able to dispose of the POWs any way he wants. He can take anything he wants. This is the worst scenario.

The pragmatic solution that will save the most lives and produce the best outcome for Ukraine is to go to the table and agree. If Putin takes "Yes" for an answer it's over. And if he piles on more demands or negotiates in bad faith they can always go back to fighting.