The intention was not to create tax shelters for the ultra-wealthy. Hence the income limits.

Thiel used a loophole to grow his wealth using the Roth as the vehicle.

He didn't use any loophole. He didn't make more than the allowable yearly contribution, which is small. If someone can turn that into billions good for them. The vast majority of wealthy people could not, hence why this is a weird anecdote like OP was saying, not an actual problem with tax code. And there are many such actual problems we should be focusing on.

It was at least not a public intention, but I know more than one multi millionaire that has used this "loophole". Peter thiel isn't some super rare exception

Backdoor Roths are a thing. The income limits don't matter in practice. The trick is getting appreciable money into the necessary accounts in the first place.

Is the loophole investing well?