I don't see what the issue is. The user could then select Apple (or Mistral) for strong privacy or another provider for customers that don't care.

I primarily want Apple to provide extension points so that I can select my own provider, just as I can choose where to host my mail or install another app as my instant messenger.

Sure, I could install another provider's app, but it wouldn't have the same integrations, similar to how an instant messaging app would be less useful if notifications were limited to iMessage.