Drivers and firmware blobs.

The real problem was never solved to begin with: all mobile devices require proprietary drivers to function at all. Because these drivers are proprietary, the only people in a position to make them compatible with an OS are the manufacturer's dev team; and they are only interested in compatibility with Google's proprietary Android fork.

When Google starts to release versions of its proprietary Android fork, any open Android fork (or other alternative OS) will have to reverse engineer that proprietary Android fork in order to match its compatibility with proprietary firmware blobs. This will need to be done for every device.

Imagine trying to find your way through a building while wearing a blindfold. It's much easier if you are able to study the original floor plan that building was modeled after, even if the building itself has a modified design. Google is taking away that floor plan.

The situation is already medium-bad: it would be trivial to use an alternative OS if drivers and firmware were open source. It would be relatively easy if drivers and firmware had open specifications. It's difficult, but feasible in the current situation, where drivers and firmware are closed spec, but designed to be compatible with a close fork of an open source codebase. It will be extremely difficult (and technically illegal in the US) to do when drivers and firmware are closed spec, and designed to be compatible with a closed source codebase.