It is an open protocol.
There will be several custodial Nostr clients, it’s only a matter of time.
They may unfortunately be even be more popular than non-custodial someday.
The key is to make the experience better on non-custodial apps than on the custodial apps.
Email works better custodial, web hosting works better custodial, there is another chance to overcome those two issues and more with making Nostr better as non-custodial.
That will be possible when we acknowledge the serious user experience issues with non-custodial and work to constantly make the experience better than custodial.
ZBD solves many experience issues for its users and ignoring that is doing us a disservice.
Many people using ZBD don’t care about its shortcomings (they’re coming from old social media). They will either use custodial or they will hopefully find a non-custodial client that compares to ZBD when one exists someday.