The problem is silicon is really cheap compared to germanium, so germanium only makes sense if the increased speed is worth the cost (which it probably isn't for almost all applications);
What’s the cost factor? Ie why not have a UI germanium chip and silicon the rest of it?
He's talking about germanium channel FinFETs, GaAs was intended to entirely replace silicon, which probably was never going to happen.
GaAs logic is probably happening at some point, just not yet. All improvements like that which would be incredibly expensive to develop will be held off on until all cheaper options have been exhausted. It does seem to be slowly moving though.
GaAs is gallium arsenide, which still has applications in high frequency transistors.