I agree that MUP is kind of broken in its current form. But I can't say that I like this idea as a suggestion to improve it.
My general play style of the game is that I'm only interested in training slaves that I want to keep. I have no interest in spending my time training unwanted slaves simply so that I can steal their AP and convert them into MUP and then sell the slave. Or, as is more likely, simply have a bunch of unwanted slaves lingering for ages in my jail or my mansion while wearing handcuffs as I completely ignore them until the game allows me to steal their AP.
For me, the ability to steal AP from every beaten bandit on the road and convert them to MUP (and thus basically erase the MUP system from the game) is the only thing that makes the MUP system bearable in its current format. If a player wants to gain slaves, train them, and put them all to work earning food and gold and maintaining the mansion, that player would never be allowed to upgrade their mansion at all (Not totally true. One can sell slaves to Sebastian/Umbra and get MUP based on the slave's grade, which I had to do before AP -> MUP existed or before I found it. But that equates to so few points and such a relationship penalty that it takes forever to manage enough MUP for even a single, simple upgrade.).
I can understand the desire to make the MUP system more meaningful and more integrated into the game. But if the MUP system is to have its primary source dry up, new ways of obtaining them need to be opened up (something that does not force players into a single style of play, because upgrading the mansion isn't an "optional" add-on.).