Again, nothing preventing Apple from doing exactly the same thing with the hypothetical 10.8" iPad. Similar outside shell, different internal layout.
They could even just leave the extra space empty or fill it with plastic or metal spacers if they so desire. Note that the Air 3 only has stereo speakers, not quad. It seems like Apple put some plastic spacers where the top speakers were located on the Pro 10.5.
Apple increased the size of the basic iPad to 10.2" so it will be compatible with the Smart Keyboard that the Air 3 and Pro 10.5 use. I honestly don't see much point in them changing the shell again unless it's for a similar purpose (Magic Keyboard and Apple Pencil 2 compatibility). Having the same accessories shared across the various models would also make iPad upgrades and the decision to buy these expensive accessories easier.
Instead of using the 10.5" chassis, Apple chose to enlarge the 9.7" chassis to fit 10.2". That suggests the cost of enlarging the 9.7" chassis for 10.2" was still less expensive than reusing the 10.5" chassis. Not only that, the 9.7/10.2" chassis contains a bigger battery compared to 10.5". I don't see the logic in using or reworking a 10.5" chassis given all the downsides. If Apple intended to do so, they would have used the 10.5" chassis for the 10.2" model in the first place.
Apple accessories are extremely high margin and have limited production runs. Most people buying the $329 9.7/10.2/10.8" iPad aren't likely to have a Smart Keyboard to begin with. Again, I don't see the logic in using an ill-fitted chassis with spacers just to accommodate accessories. The ratio iPad buyers to Apple Smart Keyboard buyers is probably something like 8:1.