You need more than bootloaders. You need drivers compatible with somewhat maintained kernels too.
My phone's bootloader is unlocked and if I fix my broken USB port I could flash any Linux system onto it, but Linux has no drivers for most of the phone's hardware. The WiFi and Bluetooth probably won't work and the GPU probably won't be doing anything useful and the camera or fingerprint scanner will never in a million years perform like they do on Android.
You can't really continuously operate a smartphone without disconnecting the battery or it will swell up and die. Doing so with this phone will require careful removal of both the screen and the back plate with heated pads, suction cups, spudgers, and lots of tiny screws.
I may one day get around to replacing the battery and bad USB board, but if I do so it'll take at least a day of YouTube-guided work, maybe more. Then I'm still stuck with an outdated version of Android or an unofficial LineageOS fork by Some Guy because projects like postmarketOS have so far given up on my device (one maintainer broke their phone, the other gave up "for their sanity").
Until parties like Qualcom start working with the Linux kernel instead of against it, and Apple opens up their bootloaders, I don't expect these devices to be usable for anyone for the next couple of years. It sounds quite enticing to have an ARM cluster of relatively quick trashed phones, but the work you need to put into each phone separately to get it to work just isn't worth it.