i was thinking the same thing..
the only reply to adding more prestiges is people say, "OH DERP DERP DERP, can't cuz can't add mwore squad mumberz duhhhhh."
well ever think that they didn't have to add a squad member to add a prestige? make it a "buy prestige" options in barracks and make each prestige unlock to cost w/e amount of SP they wanted to make it... like 1,000 sp for p11, 2,000 sp for p12... and so on. and then, yes, your squad members will earn the levels across the board as you do with the prestige so they don't fall behind.