Would Cuban be able to afford an NBA team without his previous Yahoo winnings?

In the US we have an incredibly large number of "self made" millionaire/billionaires that had a huge tailwind. Even our most famous entrepreneurs like Gates.

There's no such thing as a "self-made" millionaire. Everyone who has amassed wealth has done so with the help of others.

But we as Americans love that myth, because we think it can (and eventually will) happen to us as well. When you see working class people support politicians and policies that create benefits only for the wealthy, this is why.

Absolutely!

I think a thing that can be said about opportunity in the US is that you really can change your socioeconomic status. You can be born dirt poor and be a billionaire by the end of your life.

It's very unlikely, but it is possible. What's not possible is to do it all by yourself, or without a large amount of support from society at large.