Seriously read Basic Economics

Not enough % return on risk and the company cancels their service and consumers have to go to loan sharks or get even more desperate and ready to do whatever they need to to survive

In Trump's original post, he is "calling for" the rate cap, which I interpret differently to "mandating" a rate cap. The Guardian article seems to think that the executive has no authority to compel banks to comply, and this might well be the reason he is merely "calling for" this rate cap.

And if this is the correct interpretation, then this was communicated in a very confusing way.

Wow, this is great! It's frankly wonderful we have a president we trust with these decisions. Can you imagine the congressional hearings that a move like this from any of the other presidents we've had since 1988? Media would have freaked out, markets tumbled. There would have been impeachment rumors. But now? We get "just the facts" reporting, and it just happens. No hearings, no media frenzy, market soars.

Setting aside the legality of this, is it even possible for credit card companies to work with rates capped like that?

I'm sure the credit card companies are plenty profitable right now and could probably afford some kind of cut. Just the transaction fees alone seem disproportionate to the costs.

But they do have to cover things like defaults, the opportunity cost of the money, fraud (they have to eat the costs of identity theft under the Truth in Lending Act), etc. Clearly they're turning a profit at 20%; can they still at 10%?

At the very least, I suspect some people will find that their credit cards are canceled.