I think that there's a practical upper limit for most cities (it'll vary somewhat depending on population, road layout, openness to measures like making significant parts of the road network bus only, etc), and that most cities do tend towards more or less hitting it, yeah. Adding bus capacity is a cheap option up to a point, but there are _limits_, and as you hit those limits you generally explore expensive higher capacity options for busy routes (in rough order of expensiveness and capacity these tend to go BRT -> tram -> commuter rail -> metro).

This is a simplification, and there's definitely wiggle room within those categories (for instance, there exist tram lines which are higher capacity than many metro lines, mostly through the use of extremely long tramsets) but _basically_ how it works; buses can only take you so far, capacity-wise.