Actually if Aero got head smash it'd make Mega Aero useless, kind of like how Magic Guard on Alakazam makes Mega Alakazam useless.
There would never be any reason to ever run Mega Aero, ever. Head Smash without Recoil is such an incredible move on its own that you would never have any reason not to use it. I'm saying it's so good you could use a banded Mega Aero with ONLY Head Smash, not even giving it 3 other moves to work with, and you'd suffer no ill effects. Mega Aero only gains in advantage in being able to switch moves, which is completely negated by the fact that Band Aero using Head Smash is better than anything Mega Aero could possibly accomplish. You're much better off running Band Aero + another Mega than running Mega Aero too, since Band Aero can soften up or outright kill walls to allow another Mega to sweep, instead of just having Mega Aero kill itself in its own recoil. There is zero niche is wasting your mega on something that's going to take down 1 or 2 Pokemon and die to recoil while doing so, when normal Aero has practically the same damage output and take down 1-2 things while still being alive afterwards. You severely underestimate how bad the recoil on Head Smash is, the value of a Mega Slot on a team, and greatly overestimate a minescule increase in damage, as well as the ability to switch moves when you could potentially have a move so strong that it hits just as hard or harder resisted than your other neutral or super effective moves can hit.