How about an alternative proposal: scrap courthouses and simply give everyone a GC reduction modifier or 25% at admin 8 instead? In practice, it would achieve almost the same effect in practice because it would remove the one strategic decision most people ever make with buildings which is related to the courthouse.
No? Bad idea? Then why are you advocating for the exact same effect by a different method. The only times I find myself caring about the building slot limits are when I need to remove stuff the AI has built to fit a courthouse in, or when I find myself having to choose between a courthouse or a manufactory. I suspect, in practice, this is going to be the same for most players. The first is a straight up flaw in the game/UI. The second was a large part of the point in the governing capacity modifier being attached to the otherwise utterly unloved courthouse.
Without slot limits you build manufactories and courthouses everywhere. The exact order is a minor decision, and for the most part isn‘t even a decision. Slots do introduce strategic decisions, unless you are so resource constrained you aren’t building much anyway.
This is not a bad idea honestly.
I actually don't like the current iteration of EU4, once you get to courthouses and state house, the game turns into micromanagement hell where you have to babysit every single province to make sure it has the optimal buildings. This has become a major issue that arises from the otherwise decent idea to give the player more so-called 'strategic choice'.
Except when your empire literally consists of a thousand++ provinces, it's a goddamn chore.
I spend FAR MORE than half my time playing EU4 these days dutifully clicking on every single province of my empire one at a time and going through my internal list:
No state house in state? Build one in either gem/glass/paper or the 'worst' province in the state.
Fish/grain/livestock/wine? Courthouse + soldier's households + barracks (+regimental camp if possible).
Anything else? Courthouse + manufactory + workshop (+regimental camp if possible)
Once that is done, start adding things like temples, shipyards, extra barracks, extra workshops, maybe a marketplace if it's a high value trade node.
And since 1. tech, development and ability changes throughout the game and 2. my memory isn't infallible, I find myself going through every single province every few years, of my entire empire, deleting buildings and building new ones and fixing mistakes in buildings or deleting crap the AI built.
Simply from the viewpoint of removing a MASSIVE chore from the game, removing building limits is an excellent idea.