Even though that's kinda sad, I don't expect to see any (relevant, mass-market, used by the general public) truly open alternative in the foreseeable future. The market just doesn't seem to be there for it.
On PCs, already pretty much no one uses Linux outside of tech bubbles (and even there, seems like it's behind both Windows and macOS [0]), apparently somewhere around 1-2% [1] among the general population; in practice, it feels like it should be even less, at least based on what mainstream websites I'm familiar with see. And this is for general-purpose computers, where an open OS might have tangible benefits for the general, non-developer/ops public (not sure which, but I guess such a case could be made) – but phones don't seem to be general-purpose computers for most; it seems they're used for media consumption, taking photos, communication, maybe some light gaming, and not much else – all thinks where the existing walled gardens offer a very compelling experience with assurances for security and quality that open platforms might have a hard time delivering.
A truly open smartphone platform would probably face an even harder uphill battle than desktop Linux, and that's already failed to gain traction time and again. You'd need compatible apps for it to gain traction, but who's going to make those? It'd have to offer a really polished UX, I don't see many people picking a clunky, ugly, bug-ridden phone based on GPL vs. proprietary – lots of people (including me) buy iPhones in part because they enjoy their design and UX. How do you make sure apps aren't scattered across lots of malware-infested app stores? How do you make sure people don't inadvertently shoot themselves in the foot all the time if you don't wall them off from choices that might lead to that? But how do you wall off people on a platform that is truly open? And so on, and so on. Unless some tech giant decides their interests align very much with a truly open and relevant mobile OS, I have a hard time seeing it happen – and even then.
The current tech world is much different from when the major desktop environments solidified; this time around, the vast, overwhelming, huge majority of computer users are not enthusiasts or IT professionals anymore like in the 90s, but completely uninterested and uninvested in the tech apart from having it work for them to do relatively simple tasks, similar to what most people outside of the relevant enthusiast bubbles feel towards cars or stoves. I don't want to run Emacs on my stove and maybe blow myself up in the process, I want it to work reliably and safely and just heat my food. I'd actully like no one to be able to tinker at will with safety-critical software in their cars, because their segfaulting Formula 1 brake patch they got from some Facebook page might mean they plow into me and I die.
[0] https://insights.stackoverflow.com/survey/2020#technology-de...
[1] https://gs.statcounter.com/os-market-share/desktop/worldwide