Aluminium is a better thermal conductor than plastic? Might help move some of the heat away from the face and take some of the thermal load off the fans?
A lot of Apples decision seems to make more sense when considering how incredibly powerful (and thus hot) it is. The external battery pack (which is a 3 cell 13V pack btw) is definitely a consequence of that, they supposedly have some of the power electronics in there which helps move a bit of the heat generating stuff away from the face.
Imagine if you wanted to make a powerful headset without a fan. How would you even do that in plastic? You’d definitely want to transport the heat out to the perimeter or front, and plastic would just not be the most suitable for that.
So maybe that’s the path they’re looking towards? They might eventually be able to get rid of the fan with better chip technology, without compromising on compute, but then they need some metal to shed the heat from.
Perhaps the development they’re doing with titanium on iPhone is also a step in that direction. Letting them use metal without as much of a weight penalty.