Yes, and the C64's colour RAM is a good example, because accesses to it come for free on top of the 64 bytes/line from the main RAM. But more chips is exactly t

Yes, and the C64's colour RAM is a good example, because accesses to it come for free on top of the 64 bytes/line from the main RAM. But more chips is exactly the sort of thing that would make it more expensive. And I assume it would also have been more complex to build, or something - because if it were cheap and easy, surely people would have done it?