If they hold money in their Stripe account, it would transform Stripe into a money transmitter. The regulations involved are completely different and immense and would change Stripe into a different company. Also, the regulations are different for every state they're in, with different requirements on the board of directors (like NY state). A lot of the things they do as a processor right now would be impossible if they turned into a money transmitter. They would turn into Paypal not because they want to, but because the regulators would force them. Including everything that's bad about Paypal.