Some of the bases with the mega towns on the crater map already do have manned 17 LBS.
The mega base (and only the mega base) has a grand total of
one 17 lber in town. It is entirely and irretrievably useless. Because it is elevated and can't shoot downwards, and because it is surrounded by buildings and trees, it has no shot on anything. OK, not entirely true, over its 360 degree FOV it has a shot on a tank in maybe a total of 10 degrees, but only if the tank is far enough away that you don't have to depress the gun (it has a tiny negative angle of depression).