The ability to setup services to DM you over nostr without using email, proprietary platforms and services, or need to signup for an account with email or phone number for api access cant be overstated enough.

The real limiter is going to come down to whether enough relays permit writing events without an account.

I cant justify paying for relays for each npub. But i would be open to plans where a master npub pays and can have X number of additional configured pubkeys of up to Y messages per month. Some bot/services are routine and predictive in volume of messages while others are reactionary to user input and could have more volume. The latter kind are more reasonable to need a separate paid relay account