I think the point is we don't know where we are of the pyramid at a time. For example it's currently at 14th bank level. If the attraction of the people stops at 30th level, a person invests 1 BTC to the system 'today' can earn 1.5 BTC when we reach to 30. But if we're on the top of pyramid yet, and lets say we'll reach lvl 90 till people get bored from the system, same person can earn 4.96 BTC from the system.
I was thinking that it was so late to enter to the system, when I made a tiny investment. Today I calculated the amount of BTC at the system when I entered. And it was 4.26 BTC. After 6 days, amount raised to 26.93 BTC. That's a quite fast expansion giving me the idea of still we're at somewhere close to top of pyramid..