I mean...it was intended for everyone to use, and the intended use was to avoid taxes (in this one, very limited context). And professional investors are part of "everyone".

So...I mean....it really is the intended use.

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?

Perhaps disqualify accredited investors from ROTHs? Or make it convert if the holder makes so much $$ they become an accredited investor.

Or if the individual has paid AMT or less than 20% average tax through age 59.5 then tax all withdraws as ordinary income.

I believe that in some countries the equivalent to a Roth is limited to exchange traded companies and relatively low risk funds and bonds. If the intention is for normal people to save for retirement this makes sense to me.