No one owns a car factory; it's divided into shares. And anyone now with $20M can retire. And I don't think the problem would be that so many people hit $20M that everyone retired.

But people did own a $20M car factory. And it grew and grew. Once it hits $20M, you're basically forced to sell a part of it, and once those parts grow over, you have to sell another part, and another part...

You don't have to sell any physical piece of it, only shares in it. The company can still grow, but individuals would not be able to hoard the wealth it creates.

So let's say i have 1000 bitcoins... bought them for $0.01 a piece, and held them... now the price is around 40k.. so I have to sell half of them to keep only the $20M? Tomorrow the price will be $15k, so i'll be below again... what do I do now? The day after tomorrow they might be 100k.. sell more?

I create a bakery... it's successfull, it grows to 21M... now I have to sell a part of it... and it grows.. sell another part.. it grows.. sell another part.. what then? What if the second coowner bought a half of it, and the price has gone over 41M, do we now both have to sell a part of it? What if it loses value?

What if you own a farm somewhere, with many hectares of nice land, and someone decides to build a railway and a highway there, and that land becomes worth many millions overnight?

Hard limits on wealth are not practical. Taxing private persons for taking out and using that wealth is the solution.