It doesn’t feel so far fetched to me. People are always complaining about the same 3 technical things: VolTE, call quality and security model. Surely that’s doable? And app support, but that’s the usual networks effects problem that’s also manageable.
As to the different Linux variants, the idea probably would be to develop a canonical model with interoperability standards somehow.
Android uses the Linux kernel, maybe some parts can be migrated or used for guidance, at least. Probably not necessary.
I‘d not be intimidated by the years and money that has been poured into iOS and Android. The years are incidental, this is just the time that the companies have existed and correlates weakly with the time you‘d need to develop from scratch nowadays. They have thrown away a lot of code and years of work in the process and are likely also carrying a lot of legacy code which they‘d happily rewrite if they could start over. AI is also accelerating development. There’s also something cultural there, US tends to fund lavishly, maybe we figure out how to develop more efficiently and perhaps decentralizedly, prioritizing standards and interoperability over monopolies.