The point of open protocols is not that there will actually be many providers, but that one could switch providers easily if they do something fishy. It's about the threat, and what can happen on margin. Keeping a sword dangling over the monopolist's head is reasonable.
Plus, if these protocols are built on top of cryptographic methods, it might not be so easy for the providers to do something fishy in the first place.
Relying on a centralized competitor to offer something better would be dubious. If they gain substantial marketshare there's nothing stopping them from deciding to simply swap back to our current system. It could even be done retroactively since everybody has those lines in their terms and conditions that amount to 'We can do anything, and you can't do anything about it.'
But with a decentralized system you have the possibility of a new type of domain where while one operator may be able to be the only one that can create them, once they transfer that domain to you it becomes literally impossible for them to access, change, or affect it in any way, shape, or form. Decentralization is able to turn various digital goods into something much closer to their physical counterpart where you are buying something that is no longer connected to the seller after checkout. And this can really help ensure the integrity of services that otherwise might be susceptible to bait and switch business models.