Arbitrary isn't an insult, if there's a satisfying rationale for it go ahead and say so! There is already a very low limit for settlements, so changing what Aqueducts do, for example, would allow all cities to be flattened, so you never end up with actual cities. It doesnt seem as satisfying to me to go that route, I like having the option for larger cities to be viable, but not game-breaking though.Why is the first bold arbitrary and not the second? How would you put a satisfying number in that #? What's stopping me in putting forth a rationale of my own in the number of pop cap? I mean, now that I look closer to your proposal, it's really great for a "build and forget" approach. Since there is really no point in growing cities, then every city you conquer just need to make sure it conforms to one of the building templates and just forget about it.
In terms of a satisfying number for the number of Pops a building can service, I'd just put that down to experimentation until the game balance feels about right, generally aiming for a # that means you can't have a Jack-of-all-trades city.
As for "build-and-forget", I'm mostly comfortable with that. This game has a ridiculous number of territories to manage, and as a GSG I would not expect experienced players to spend a ton of time deliberating for each. There's a reason macro-builders are popular.