Antitrust action is badly needed in this area. It is ridiculous that I need permission from my device manufacturer to install software on hardware I own. There is no viable alternative than to live in Apple and Google’s ecosystems. This duopoly cannot be allowed to keep this much control of the mobile platforms.
There needs to be a mandatory override for any lock down put in place by a manufacturer. I understand the need for security, but it should be illegal to prevent me from bypassing security if I decide to on my own device. Make it take multiple clicks and show me scary warnings, that's fine.
Technically Android still allows installation of anything if you use the debugging tool. Maybe that is where we have to draw the line, I'm not sure.
Aside from everyone pointing out that you can't do that on most phones, there's also the fact that installing your own OS will block you from using many apps that check your secure status.
I've been wanting to get into OS dev for years now, I may make an attempt at it soon. When I was younger I built my own kernels for the early OnePlus phones. Maybe I can build an alternative to Android, doubtful but I like a challenge.
The hardest part to making an alternative is the app ecosystem, you almost need a complete suite of 3rd party apps built before you can get any initial adoption.