So here is something that is often done away with "its not possible, it cant be done" but I dont get it...
It's not that it's impossible, it's simply very very very niche and realistically nobody cares about it.... least of all "phone enthusiasts". Ok, maybe 5 or 6 people care about it worldwide, but those are mostly paranoid uber-nerds that want to run freebsd on everything.
You are either very young, or you've missed the whole fun over the past 10-15 years. We already had all that: phones with Ubuntu touch, dual-boot tablets, dual-boot phones, windows on phones and tablets, linux on phones and tablets, android on desktops, etc.etc.etc. The thing is - there is no demand for it, so independent devs simply stopped porting stuff to phones. Heck, I still have an old ASUS Vivotab that runs Debian (I still want to use it as a 3D Printer brain/screen, once I get to rebuilding that monstrosity), but outside that niche use case - it's just collecting dust on top of my server rack.
On modern phones it's a bigger problem, since you have various security features that depend on bootloader being locked and phone not being rooted. The reason why it's like that - is the wild west of early 2010s, when you could easily buy a cheap chinese Android phone with root and enabled USB debugging right out of the box, and you were lucky if it did not come with malware as a bonus. You can flash an alternative firmware and keep root access, but you can't use any banking/payment apps or like in my case - local apps with my digital ID and govt services. Plus most modern phones have things akin to SecureBoot, and even more pain in the ass to get around it.
There's also a case of hardware variability between phones. You may have two phones with identical specs on paper, but in reality each one has its own device tree with its own can of worms. Different captouch IC - no touchscreen for you. Different gyro - no autorotate. Custom proximity sensor (or lack thereof) - screen will be stuck on or off. etc. etc. etc. So, if you are making, let's say, a linux distro or custom Android build for a phone - you are usually making it for a very specific phone model.
Another big problem - lack of drivers and hardware documentation. It's slowly changing, with various ARM chipsets going to laptops and chromebooks, but back in a day an SoC manufacturer may only put up one version of the "open-source" driver that's not really open-source and has many prebuilt binary blobs, and because of that it will only work on one specific Linux kernel. That's the same reason older phones had much smaller update cycle.
Lets take Microsoft OS, Windows, I can install Windows 10 on ANCIENT hardware, like a Core 2 Duo or even a Pentium 4, this is perfectly normal that you can do that.
You can doesn't mean you should. At some point even the most stubborn software companies begin to implement features that may use newer hardware features (as an example - FMA or AVX instructions), and things stop working "smoothly". That's why back in a day I had to give up on my Westmere rig: it was still usable, but some things were orders of magnitude slower than even on the cheapest at the time Core i3-6100.
. Few times I thinking of upgrading but phone nowadays are boring. The only requirement I wanted it to have is 3.5mm jack. Only flagship I saw that still have that is Sony Xperia. I might buy Xperia 5 IV or 1IV, not the latest VI mind you, usually older flagship is cheap and gave 99% of what the new flagship offers.
