"debugger vendors in 2047 distributed numbered copies only, and only to officially licensed and bonded programmers." - Richard Stallman, The Right to Read, 1997
I only have Linux PCs (laptops) and servers, 100% of my work and personal stuff is done there (though for work I do need to hop into MS365, Google Workspace, Zoom, etc, hooray for browsers, my final firewall between me and the walled gardens, though we can have a whole discussion on that).
For mobile, we have PostmarketOS, Phosh, Ubuntu Touch. I really must try living in them, is it on me? IDK, our government even has an identity app for iOS and Android. I should not be using it, I should stick to web. But its so much more convenient. I'm just weak, aren't I?
Maybe I should go for Ubuntu touch, with an iPad on the side or something. At least my most personal device is something I control then. Or just keep my Linux laptop handy (or make a cyberdeck!). But I want a computing platform that does not require carrying a bag. It's kinda sad. Even GrapheneOS (one of the most personal and secure mobile computing experiences out there)'s future is in the hands of its greatest adversary, the one that does not want you to have a personal computing experience.
In a way it's not. As you mention, we have several of them. But they won't have mass-market appeal until they can run the same sorts of apps that Android and iOS can run. And no, "just use the mobile website" is not an answer.
How do I deposit a check with my bank on my phone without the app? I can't; the mobile website doesn't have that functionality. How do I send someone money via Zelle without the app? I can't; the mobile website doesn't have that functionality.
How do I use contactless payments? I can't; the ability to build an app like Google Wallet or Apple Pay requires deep pockets and trusted payments industry connections that open source mobile OS developers will likely never have.
How do I use Google's productivity suite? I can't; the mobile websites aren't functional enough. How do I use Microsoft's? Ditto.
How do I use the remote-lock functionality of my car? I can't; that's only available through the Android and iOS apps.
I could go on, and on, and on, but I think you see the point. Many people who advocate for these alternative OSes don't get it. "Do you really need that functionality?", they ask. "Why can't you just do that stuff in a web browser on your laptop instead of on your phone?", they ask. "Just use a physical credit card like I do!" And then they wonder why their alternative mobile OS will never go mainstream.
People actually really care about those features and capabilities. It doesn't matter if the people who build these alternative mobile OSes don't care, or think they're stupid, or unsafe, or bad for privacy, or whatever. If you don't build what people want, they won't use your stuff.
Emulating Android sufficiently well enough to run Android apps is a decent start, but so many apps rely on Play Services and Play Integrity that it's a losing battle, or at best a cat-and-mouse game to keep things working.
On top of that, mobile chipset BSPs require financial commitments and being a Real Company. Most open source outfits can't cross that bar, and the likes of Qualcomm will be wary dealing with an organization that wants to do open source.
This is where antitrust laws are supposed to come into play. Play Services are a pain but in principle you can implement alternatives to them. It's the attestation stuff which is aggressively anti-competitive -- literally setting up a system with the primary function of excluding competing implementations from compatibility.
We can't let corporations get away with the fraud that competing with them is a security vulnerability.