Already against Diamondback rockets, Some bosses are impossible to win against highly mobile classes, Not to mention Flying ones.
people Stopped picking soldier mg because of Diamondback spies, Spy deals more damage, Runs faster, Can keep up the rampage by walking rather than using another rocket, Spy's Survivability is way higher than Soldier's, Whether it was for the kunai/DeadRinger/BigEarner.
Adding Demoman is imo, Unnecessary.
We really don't need that many flying classes, If they lived enough to be the last standing, Teleport bosses just won't catch a flying demoman, Not possible. Especially in certain maps with a higher places and health packs. Basically by unlocking a mg demoman, There will be more needed changes, Debuffs and buffs, And still won't be that fun.
Like I said in-game, Might aswell giving MG to the whole squad really, Turn the gamemode into angry birds
waiting for my Heavy MG