If indeed his Generals didn't anticipate that, they are not worthy of their titles. However, I think this is all in your thoughts. There isn't really any solid evidence that Putin predicted and his Generals predicted Ukraine would fall after only a few days. Surely they would have expected insurgent fighting to last years even. I mean they can look at US experience in Vietnam, Iraq, and Afghanistan for a start to realize occupying Ukraine would be costly and take a long time. Putin must have some sort of plan and he probably wanted the favorable outcome to be gained in his pushing Zelensky towards negotiations.
Well, if Putin original intention is to force a regime change, then he would have to aim at capitulate Ukraine as soon as possible, the longer it got drag on, the more time he is giving the Ukrainian to organise a defence. And you also want to do it with minimal civilian lost. So the take over will not be opposed.
The Russia battleplan echo this fact, first we see the amount of troop Russia use, it pointed at a surgical assault, not a general assault, a quick in and out action with minimal occupation force (You usually need 2 to 3 times the invading force to occupy a city) Also the diversion of 3 avenue of attack which amplified the supply and logistic requirement as they are all 100 of km apart, which mean they want to take multiple city simultaneously which also point to quick capitulation. And finally the way the initial assault carry the troop and the forward deployment of troop also point to the fact the battle plan is for a quick victories.
I am pretty sure if Putin plan originally was to face an insurgency, he would have used more than 200,000 troop for this. He would probably bring along around 5-600000 troop with the majority of those National Guard troop trained with Riot control, This was not the case tho.
Another issue is Russia intelligence and counter intelligence operation. For starter, the fact that US and UK broadcasted Russian battleplan before Russian launch their attack is a good indication that Russia have a counter intelligence problem, and the fact that they follow thru the invasion with the original plan that US and UK announced before hand would suggest an intelligence handling problem , set aside whether or not Putin expect Ukrainian would welcome them with open arms is another issue, but the first two already exposed the Russian intelligence problem without the need to go to underestimate the Ukrainian willingness to fight.
And finally, you cannot force a deal if you do not own any city, you simply cannot trade road or bus junction for a more favourable deal on the negotiation table. Say for example, if Peace talk is going on tomorrow, there are virtually no bargaining chip for Russia to demand anything from the Ukrainian, at this point in time, they hold 2 medium to small size city (Kherson and Melitopol and Kherson is actually under counter attack for now) You can't really go on for a bigger piece of the pies if they were to negotiate. I don't even know if it will change the calculus if they took Mariupol because that city is now in ruined, it does not worth anything.
So with all of this put together, one can assume what the Russian expect is not what we saw here on day 24. If you compare this to US invasion in Iraq, by Day 24, the US had already took Baghdad, and Russia still have not start the fight or even bombard Kyiv yet. What you are looking is street to street fighting 3 or 4 times the scale of what we are seeing in Mariupol and Kharkiv, where both city still remain in Ukrainian hand. You are talking about 5 to 6 times the current casualty number ahead.
