I dunno, I would much rather retain the ability to download a new browser that promises improved privacy (i.e. by integrating with their own DNS service that includes pi-hole) than be limited to one of four well-known American OS vendors.
Alternatively, I see no reason why someone can't write a small application that runs a resolver on localhost, adjusts your hosts file accordingly, and forwards queries to any third party or distributed network of your choice. Wait a sec, I already have dnsmasq and/or systemd-resolved doing this on some of my machines. It can be done right now with no need to wait for the OS vendor to change things.
You really wouldn't be limited, it would be no different than time.microsoft.com being the default on Windows or rhel.pool.ntp.org being the default on RHEL.
The local resolver that proxies DoH or DoT is nice (I'm using it right now too!) but it doesn't solve the problem of applications wanting to be sure they're getting an encrypted connection.
However, the "centralization problem" isn't solved by more competitors existing because widely deployed applications will still have to choose a single option for all but a tiny segment of users that will change it.
If the default comes from the OS vendor then we're in a slightly better place since the default for macOS and iOS will be Apple's servers, Windows will be MS, Android will be Amazon or Google. Competition in this space comes from the people who decide what DNS service to use, not the people standing up the service.