Resolving is already centralized: your ISP has 100% control over what you resolve, and you can't do anything about it. This is doing the opposite - by securely implementing resolution in a user agent, in a tamper-proof way, under the control of the user.
Every user is free to run their own resolver or use any of their ISPs or third parties, which is pretty close to the definition of something decentralized.
Except, as others have pointed out, there are documented cases of ISPs hijacking DNS traffic, even for people who have configured their client to use resolvers other than their ISP, which is possible because of DNS's lack of authentication or encryption.
Besides, I don't see how adding an option for DoH to Firefox is centralizing anything, you're free to set the DoH URL to whatever you like, and you're free to run your own DoH resolver, just like you're free to run your own vanilla DNS resolver.
Can we please go easy on the newspeak? Centralizing resolving to a handful of actors will not improve privacy for the most part of end users.