Nothing lasts forever. The rich get most of the economic growth is inevitable. It's the Matthew effect. In general, the more assets you have, the more passive income you have, which in turn frees people up from worries about making breads for the family, and thus can spend more time on think and do things more important in the long run. Again, that would reinforce their financial status. In the meantime, the poor would have to worry about next meal, and mostly don't have the reserve for investments. Thus, they will most likely struggle to save something, and even if they do manage to save some, an unexpected event can easily wipe it out before it reaches the threshold.

If the riches were conscious enough, they would return a fair share to the poor and the eventual crash will be deferred much longer. But you know, everyone wants more money, no exception for the riches. It looks the only thing we learn from the history is that we learn nothing from history. So here we are: the same drama of empires' rise and fall, the only differences are the locations and the actors.