EIS is more akin to a poor-man's gimbal than it is to OIS, and that's the purpose it serves in GoPros. One could argue that for sufficiently small range of movement a gimbal and OIS can serve similar purposes for stills photography - a gimbal after all is a form of external stabilization. However EIS is not seriously applicable to stills photography as it does nothing for a standalone capture and OIS cannot replace a gimbal (or EIS) for video capture other than in very narrow scenarios where static support would generally be a better solution.

It's obvious that OIS + EIS > EIS. However it's not that case that OIS > EIS for all applications. One could argue that given just OIS one could apply EIS in post, but that's just a different implementation of OIS + EIS.

As far as I know, the GoPro is fixed-focus - there's no moving parts in its optical system. Adding mechanical stabilization whether optical or sensor-based would reduce the durability of a system of which one of the main criteria is being able to withstand repeated impacts. Maybe it's possible to build a sufficiently ruggedized OIS solution, but it's not clear whether the expense or size/weight of that would make it viable for action cameras.