If this weren't the case, wouldn't the US economy be dominated by Astors, Vanderbilts, Morgans, Rockefellers and Carnegies? While some families still maintain some wealth (eg Rockefellers), it's nothing like the relative wealth from whence it came.
Trusts expiring aren't the reason. Perpetual trusts won't lock in generational wealth forever. Human nature just gets in the way.
Trust administrators can be corrupt or just bad, determined trustees can dissolve the trust, wealth gets diluted across generations as the numbers grow and of course just being given a huge pile of money is no guarantee you have the mindset, skills and luck to maintain it.
All of this is good. The ultimate form of wealth redistribution is war and revolution. Wealth inequality seems to be ultimately self-correcting. I just don't believe that in 200 or even 100 years the world will be dominated by Bezos's descendants.
While there are clearly problems with inequality now, the simple fact is that there is no better time to be a human than now compared to the entire of human history for the average person by pretty much any objective measure you can reasonably choose.