The question is often how closely they let apps compete with them. Ie, can I roll out a (cheaper) invoicing solution with for example a $5/invoice cap. Quickbooks et al do allow this. Some players do not like that.

If history is any indicator, Stripe can always Sherlock the service and build a better native feature.

It's almost like any successful developer needs to develop an app thats popular but not that popular and verging on slightly more difficult so that the moat holder (in this case Stripe) doesn't want to build it themselves. Alternatively make something that catches fire really quickly and sell very quickly before they can build it themself.

Welcome to capitalism. These are shark infested waters. Either be a shark or learn how to swim safely ;)

How is that capitalism related?

Unless there's a reason for an app to be exclusive to a marketplace, then I disagree that it deepens the moat.