AFAIK some things that are (or were, I may not be up-to-date there) present in the megi's tree aren't even upstreamable at all, like the modem power manager driver. When you maintain your own tree, you can be much more liberal in what goes there compared to what mainline maintainers will accept.

Because it isn't that much work to keep rebasing his tree according to megi.

Could the pmOS folks get involved and start submitting mainline patches themselves? The Mobian folks might also be interested since they probably want to minimize long-term deviations from Debian, which is based on the mainline.

We're working together with the maintainers of various distros now to first of all avoid this mess on the pinephone pro, and then on the longer term fix up pinephone in mainline.