Not saying I agree with the state of things, but very few people use open source apps other than a few high-profile ones (Chrome, Firefox, Telegram, et Al).

We need open source alternatives to everything for AOSP to be truly viable again.

LineageOS with F-Droid practically achieves this, but Google's contracts stop OEM from distributing this. The same situation from Microsoft and Linux a while back.

You or I may be able and willing to buy OSS phones, but the average consumer never would common proprietary software (FB, YouTube).

OEMs would never bother selling them in the first place, obligations or not.

Can't you redistribute NewPipe, and the whole shebang? I agree that selling phones with AOSP/LineageOS only is worthless.

For Facebook, you can make a custom Firefox bookmark, or one of those WebViews hard-coded to Facebook.

You need to add microG to the mix, otherwise a lot of Android apps (including many open ones) won't work at all on your "Android".