Fdroid can never replace the main default because their strict rules rule out too much. I approve BTW, I like that a special place like fdroid exists, but it can only ever be this special niche thing.
But as time goes on and software tools become more powerful, there is just not much moat. I don't even mean AI, I just mean the ordinary power annordinary computer and the gobs of highblevel languages and frameworks and automation that just grows and grows all the time. It's not that hard to port an app to a new platform, or a new distribution mechanism, or a new clone source for an existing platform and distribution mechanism.
Big universal things with 100% capture get replaced overnight all the time. Everyone switched from MySpace to Facebook in like an hour one day. Other things like my Steam example took longer but they became at least an equal player, a real alternative that normal people could and did choose in numbers that Sony and MS couldn't ignore, pretty quickly.
Everyone switched from Palm to Android pretty quickly.
Google has a few extra levers which is agreements with all the phone manufacturers who in turn have to deal with the carriers, and google are doing the bulk of the ongoing os developement and security work so the manufacturers don't have to.
But they could just start doing that same work, or spin up a new 3rd party who's work they all share, easily, any time. They all actually already maintain half-assed backup plans they can switch to and go whole-assed if google gets that 1% too annoying one day.
There are really no easy robust moats any more. Even the biggest are precarious and have to be actively maintained through constant shenanigans.
No matter how big the capture, it's never actually very strong.