I hope some OEM (especially those opposed to google) picks up and develops mainline linux like Pine Phone.

Huawei is being forced to do it. But like Android, their HarmonyOS is not 100% open-source. There’s also KaiOS, which some Nokia and Alcatel, and all Jio, devices use.

even Dalvik and the android runtime itself is an inefficient relic of 10+ years ago when mobile devices had at most 2gb of ram and a tiny low power ARM processor.

Both the ones I mentioned are designed to be more memory efficient. KaiOS in particular is aimed primarily at feature phones and entry-level smartphones.