If you want to have missiles with 20th and 21st Century engine smoke trails then so be it. But this is supposed to be what? the 23rd or 24th Centruy? You'd think by then missile engines would be entirely different than what we have today? Maybe by then we'd have totally smokeless engines which might be ION type of engines, or plasma which by the way we do have both working engines now. Heck for all I know, it could be powered by a super high compressed gas of some sort that leaves no heat, visual and Ladar signature.
I think the bottom line is, if you like your missiles with trails, then have fun. If you don't like it, same thing have fun as you have the option of both.