I'm gonna make this quick and short.
I've just realized that the maximum infrastructure you can have in a single city is 20k. While 20k is a really high number it's no doubt that somebody can reach it easily if they wished. After that theres nothing left to upgrade and only more to lose.
I propose that there be no infrastructure cap, or if needed a very high one (100k+ Infrastructure). While most people don't go above 3k infrastructure it would be cool if you could possibly make a city with millions of population