First thought that sprung to my mind was, once the big three have made
my top ten, they'll probably have been in business for a good few hundred years anyway

since I don't recall anyone else mentioning the Westland Lynx, BAe Hawk, Renault VAB 4x4 or the Cadillac-Gage V150
So it doesn't matter what they make, someone somewhere will want it (well, almost anything!) so I don't think that lack of ideas will make them disappear.
Anyway, once they finish with the military hardware, they could always start on other stuff... construction machinery - diggers, plows/graders etc - always seem to sell quite well, and there is a whole load of people out there who collect such models who may well jump at the chance to get something bigger than the "normal" 1:50th or 1:64th they seem to get.
And there's always science-fiction and scince-fantasy as well, from exploring alien planets to the slightly iffy scince of Jurassic Park etc (and don't forget the P40 from "Sky Captain"). There's plenty of inspiration for all sorts of vehicles, equipment and figures that doesn't run foul of copyright or licencing; the Chinese company Chap Mei seems to do okay for ideas and I can't see the Big Three having too much difficulty mining the same idea sources as CM do...