Would it need access to an admin macaroon to know if the invoice was paid?
Discussion
It would need to keep an open connection to all nodes from all registered people and would turn into a very bloated thing and error-prone thing.
And easy to DoS because monitoring all those invoices means keeping state, which takes resources.
Especially when creating a zap request (i.e. binding resources on the zapper) is free, just takes sending a nostr message.
So... zaps will only ever work with big centralized custodians? That's a big let down...
Not necessarily as long some good Dev can tweak simple ln@ Foss solution.