There is a hard cap (in effect) on active users! Now "hard" is relative. It could be changed, and likely would be changed if adoption exploded and HIVE/HP price increased accordingly.

But the way the existing code works right now, there is only a certain amount of chain resource growth allowed before RC costs become prohibitive. The levels are supposedly set to ensure that operating nodes remains feasible within the limits of current technology, but I wouldn't bet my life on that actually being the case in practice (in either direction).