> "anyone can build a payment processor, nobody can get regulatory approval."

If you want to become a bank regulatory approval is hard. If you don't then its necessary but not the biggest barrier by far. Building trust with card networks, merchants & banks, interfacing with card networks, optimizing acceptance rates to a high level against the black box of card networks etc.

And then you still wouldn't make any money because the margins are extremely low on processing payments themselves - so either you have to have massive scale or have a bunch of value added services that you charge more for. For which you also need a serious commercial engine to be able to sell to either loads of enterprises or some very large ones (who will rarely sign exclusive deals - they will just give you a share of wallet so they can transfer traffic over at will).

Card networks are another story...