There's already an alternative, it's called Linux.
So really, the alternative is proper GNU/Linux (or as I've recently taken to calling it...)
See also:
https://news.ycombinator.com/item?id=50057296
I look forward to "Linux users and/or developers benefit from Android but..."
What does "Android is Linux" mean to you? It uses Linux? Why does it matter?
You asserted further up that "Android uses Linux but it has no consequences for users or developers."
That simply isn't true.
It is as close to GNU/Linux as Inferno was to UNIX System V.
Which is why Termux has to work around what is actually available as public APIs on the NDK.
Termux has to work around security restrictions in a way not dissimilar to how you would need to work around an extremely securely configured SELinux setup on RHEL.
Which is what Termux has to do, just like they have to call into specific Android services via JNI, as the NDK only exposes official NDK APIs, alongside the C and C++ standard libraries.
The alternative are other AOSP-based OSes like GrapheneOS and Lineage. Once they are used widely enough and funded enough, they could fork AOSP when needed. In some sense they have already soft-forked in that they use major and QPR2 releases, but put a lot of patches on top to fix issues, improve security, etc.
GrapheneOS has an estimated 500,000 active installs now, apparently LineageOS has 1.4M active installs. Probably around two orders of magnitude more than traditional Linux desktop-based phones, which have been trying since, checking notes, Maemo in 2005?
They are mostly relying on LineageOS now and putting their own cloud, microG, and App Store app and branding on top of Lineage. Even now they are not able to roll out Android releases and WebView updates timely. Them maintaining an AOSP fork would probably end up in an outdated, broken mess.
My money is on GrapheneOS and maybe LineageOS to do it when needed.