Ukraine

To be clear, they don't need to be sincere; it's in the best interest of anyone following Putin to unfuck their economy, get on the good side of the oligarchs, and modernize their military. The quickest and easiest way to do that is to get out of Ukraine, blame everything on Putin, and at least put on a veneer of civility until things are going smoothly for them. Even if they want to continue expanding the Russian empire, they really need to get out of the current quagmire first.

They might, if everything left is really really really dumb.

If Putin gains his own detached head, whoever comes after him will be politically much weaker. No matter who the leader will be, even if Putin, the Russia will very soon have to stop using mercenaries and must switch to conscripts. For a weak leader, it might be very difficult to squeeze people into the meat grinder. I'm not sure if such a follower-up would really have any way of continuing the war. No economy means no soldiers – except through conscription. And I don't think Russians would accept a conscription. If the fledgeling autocrat cannot continue the war, what options does he really have other than the smart one? Let go of what must go so that at least a part of the Russia will stay together.

Soviet and Russian history has a lot of points where change of leadership was a linchpin that very suddenly turned the previous heroes into detested monsters. How Russians started talking about Jeltsin very soon after Putin rose to power is actually yet another example of that. There is precedence for putting all the blame to the predecessor and then going for big reforms.

Putin will eventually lose his place. The less Kremlin has that event under control, the better for everyone outside Kremlin. Therefore, better get rolling with it, asap!
It's entirely possible that the replacement will really be as bad, but if Kremlin gets to decide on the follower, then the follower will definitely be as bad as Putin. Or worse. Because when it comes to war strategy, Putin is clueless, but micromanages all the time. If someone is otherwise the same but has some understanding of military strategy, things will only go south.

Loss of manpower is a problem, but because the Russia is constantly losing a larger share of its population, it's more problem for the Russia than for Ukraine, and therefore – a bit weirdly – the problem with loss of manpower is a net positive for Ukraine.
I don't think many people would really mind that much if the Donbas situation was reverted to the 2020 status, but Crimea is strategically so useful for the Russia in its next war against Ukraine that it's a bit of a bad idea letting go of it.

And then there's the stretch of land between Donetsk and Džankoj, Crimea. If the Russia gets to keep that, there is no way to stop them from attacking again. Ukraine's position will be so much more difficult to defend in that case than it is now, and actually even more difficult than it was in 2022.

Furthermore, although I don't think this is a subject they think much about in Ukraine, what the Russia is holding in that part of Ukraine is the best farmland on all this planet. More than half of all of the black soil on this planet is located in Ukraine, and most of the best soil in Ukraine is in the part occupied by the Russia. With the climate changing, there will be lack of food and starvation. It's either food for me, Ukrainians, and you. Or for the murderous nation east of Ukraine. It's a super bad deal if we let all that slip away. It's about the food my children will be eating. I don't care if murderers starve, but I do care if my child does.

So, at least it's a very bad idea letting the Russia have those areas. It means there will be another war in Europe, and it's better that there won't.

And then: Yeah, the Russia has set up proper defences, but how will it defend those proper defences without manpower? All of the Russian army is currently volunteers. They are volunteering almost solely for money. In eastern Siberia 50 € per month is literally a normal monthly salary. In 2015 I hanged around in Ukraine with people who got a salary of 2000 UAH, which translated to 70 € per month by the rate of those days. And eastern Siberia is poorer than Ukraine is. And those people living in east-Siberian villages, getting 50 € per month get a salary of 2000 € per month if they join the army as ~~mer~~volunteers.

The Russian economy is tanking big time. Around late summer 2022 it was prognosed that it will crash around the end of 2025 or first half of 2026. Now that's been moved to summer to autumn 2026, but the original prognosis holds surprisingly well. The same reasons that were seen back then are still valid. The whole military economy is pushed on the shoulders of the regions and banks. Those regions are now out of money and unable to generate more and the preferential loans that now form most of the loans given out by banks generate zero interest for the banks and will never be paid back. The banks no longer have money to "lend" to the military industry, meaning that the military industry can no longer sell their products cheaper than their own production costs. And that means the government will have to start paying the full price for the military hardware, meaning they will have even less money.
Paying 2000 € per month for 700 000 people will not be possible. They will have to move to conscription. At this point very few Russian soldiers go to the war against their own will. Once the ability to pay salary to the soldiers is gone, there will be only soldiers who have been brought to the front against their own will. When those start dying en masse, that will have an effect. It's one thing to send your son to a war based on false promises, but a whole different thing to have your son snatched into a war that will bring nothing to your nation or your family. It will be extremely difficult to keep the remote Siberian regions fully under Moscow's command. They will gradually stop obeying commands from Moscow.

What this means is that the Russia will seize to be able to replenish its military. Their only way of using those defensive structures is by sacrificing at minimum 20 000 conscripts per month for that. Let's tell a small anecdote: A have (had?) a friend from Moscow, whose uncle moved to Siberia. He was given a plot of land for free, a dilapidated house for free, and was paid a relatively high monthly lump sum for the first couple of years so that he could get his life going. Now, why? The reason is, they don't have enough population there. The population is decreasing so much that Moscow had to pay such high money to keep those regions inhabited. Under those circumstances, how are they going to send 20 000 people per month away from the same area?! They had a population emergency there, and they still do. Removing 20 000 people per month will not help alleviate that emergency!

It is entirely possible that the west manages to pressure Ukraine into a peace deal, but if that's not done, then the Russia will have to switch to conscription by autumn this year. And then it can no longer fight.