The question is not whether every AGI automatically prevents value drift but whether AGI that keep humanity alive are that way. We want to build FAI's.

It seems the more important question is whether AGI that prevent value drift have a survival advantage or disadvantage over AGI that have value drift.

To me it seems almost self-evident that AGI that have value drift will have a survival advantage. They would do this biologically, they would make multiple copies of themselves with variation in their values, and that copies that were fitter would tend to propagate their new drifted values moreso than the copies that were less fit. Further, value drift is likely essential when competing against other non-static (i.e., evolving) AIs, my values must adapt to the new values of and capabilities of up and coming AIs for me to survive, thrive, and spin off (flawed) copies.

Agreed. FAIs must prevent any kind of drift. That comes at a cost which penalizes FAI against other AGIs.