Today, if you own a Pixel, you can just install GrapheneOS and it will mostly work. Why just "mostly"? Not because of GrapheneOS. Because of some goddamn companies (usually banks) who want to lock you into the Google flavour of Android.
If you own another device, maybe you could install LineageOS or /e/OS, but chances are that you won't be able to relock the bootloader, so you will lose the security model of Android. For many people, this should mean that it's not a reasonable option. But that's not LineageOS' or /e/OS' fault: it's the manufacturer's fault: they are locking you into the Google flavour of Android (which many times also means their own flavour based on top of the Google flavour).
And on many other devices, you just cannot install an alternative OS at all, so you are stuck with the Stock OS.
You are essentially saying "it is fine that 99.99999% of the people are locked into their manufacturer flavour of Android, as long as we can keep using F-Droid" (because really, the changed discussed here mostly affects F-Droid, and it would be debatable whether or not F-Droid's design is fine or not).
I disagree, it is not fine. When we buy an Android device, we should be able to install an alternative OS, period. All of us.