@nazokiyoubinbou @FifiSch @grote Unfortunately, you cannot do complete backups of android devices if you are not rooted. You may have a phone in use currently, which you didn't have the foresight to bootloader unlock and root when you initially got it, and thus installing any custom ROM is not very interesting as you would loose a large part of your data. The part of the data that you would lose is unknown as app/phone backups are opaque, and might also result in losing access to some messaging applications if not done carefully. In the best case, you would have a second phone from which to transfer data from, as a reference (and for messaging apps that can transfer accounts between devices).
At least, that's how I see it. The process itself is easy, but the steps before and after are much more scary.
In all, I do agree with your initial premise that LineageOS and other custom ROMs will become more popular for those that have android headsets that have unlockable bootloaders and have the initial knowhow.
We do seriously need more alternatives to Android and iOS, such as postmarketOS, as the current state of things is getting very dire. Unfortunately, the hardware itself is not very standardised and manufacturers want to keep control to squeeze a few more cents from each device…