i don't think its about whats the devs have a right to do and not to do. i think its a matter of supporting the community around your IP and letting the community mature through each iteration of it.
If a developer could pull that off, with that goal in mind, it wouldn't bother me. But that's not the reason there are so many sequels, Devs make sequels of popular games because its a safer investment than starting a new IP.
SD has done plenty fine supporting their community without ever making a sequel. They take the knowledge they gain from making past games, and apply it to their work, but they don't have to make a literal sequel.