Let me put this way: It’s not going to happen, that’s not how it’s done, but pulling a bit back and just
doing it again wouldn’t be the craziest thing Russia could do at this point.
This is industrial warfare. The primary (not only) goal is to destroy the enemy army. Ideally with short/long supply lines (“Why are the Dnieper bridges still up?”), in favorable territory. Yes, this is as ugly a thing as it gets. Hundreds of thousands of men are dead and hundreds of thousands more will die in all likelihood. That’s how it is. And no side is innocent in this.
And, again, people don’t want to hear it but Russia is doing well enough.
The original, mostly post-soviet Ukrainian army of 24th Feb. is all but gone. More than half of their officers are confirmed dead (obituary and all), most of their original heavy gear is destroyed. Gonna avoid discussing overall casualties here but it’s a lot. People are going to be shocked when/if the numbers ever come out.
This is why in the West it’s all about begging for and donating of gear, and the training of fresh troops. Because Ukrainians need it.
And their “second army”, post-soviet gear from outside is also mostly gone. Willing donors are all but depleted, too.
And this is why, after much reluctance, Western tanks are now in Ukraine. But the West does not have all that much to give. And they are not really producing either.
So, unless things change drastically soon, there will be no “fourth” Ukrainian army.
And then Russia will cross the Dnieper again.
Or maybe not. Because the situation is much more serious than most of us Westerners realize. Our leaders are playing with nuclear fire, whether they know it or not. But not because Russia is losing. Because the West is all but at war with a nuclear power and when/if that threshold is crossed Russia is not going to wait for US forces to deploy in Poland or whatever, or worse. They don’t want this but they are prepared for it. They want to avoid it but they are not bluffing. As they keep warning us.
Finally, given the complexities and constraints of the system, “F-16s for Ukraine”, if it actually happens, will at very least mean even more NATO crews in Ukraine and in fact probably takeoffs from Poland and Romania. It’s not certain that Russia will consider this the threshold crossed, but you bet they’ll give an unambiguous response.