You think if they used another licence it would be any different? Countless open source projects have a GPLv3 + proprietary licence which is way more evil than Apache - they poison the open source with GPLv3 so no competitor can contribute without revealing their changes while they themselves can use the proprietary licence. e.g. Trolltech and QT for example but there are many others.

And frankly you should be blessed that you have a fully fledged, open source phone OS you may fork and build from. The OP wants a Linux phone OS and AOSP is a Linux phone OS. There are many forks of Android, closed and open that wouldn’t exist if Google had just decided to be proprietary from the get go. They were under no compulsion to do this but they did. If you have used LineageOS, or GrapheneOS for example then you are a beneficiary of this. You are completely at liberty to have a de-Googled modern phone OS powered by Linux right now.

Wait… they opened it so they can close it?

Android is Linux. It uses a Linux kernel paired with a BSD based user land. Also there is an AOSP version of Android which is Android without all the Google bits. LineageOS and some other security oriented firmwares derive from it. That isn’t to say Google are necessarily happy about this entirely but at the same time, they open sourced most of Android and probably see it as a useful antitrust defence and the impact of flashed devices barely more than background noise.

The issue of bootloaders is an orthogonal matter since Linux or not does not mean bootloader or not - many black box devices use Linux but you won’t be flashing them any time soon - TVs, set top boxes etc. I would argue that regardless of OS, there should be a right to repair law (e.g. in Europe) that allows people to maintain devices beyond their warranty. And if Samsung et al don’t want to do it, then they should have an obligation to unlock devices upon request.