I doubt it. Hobbyists have a habit of personalizing their stuff, esp. wargamers. It may be prepainted, but that won't keep people from repainting, reposing, and sculpting their own features. Take a look at Old Soldiers MEA, for example, or the USMC models with all the goodies sculpted on to them (can't remember who did them, sorry).
And of course, there's enough people who like the building/painting aspect to keep it going. Shoot, just the other day my wife (a casual gamer) and I had the following conversation while painting SST stuff).
Wife: I think once I finish these, I want to get some more stuff.
Me: Like what?
Wife: Y'know, Warriors, Hoppers. I think I only have, what, three of those?
Me: Yeah. Well, do you want to wait for the pre-paints?
Wife: No! I want to paint them like my bugs!
See, if you get that from a casual gamer, I don't see the hobby aspect dying. Fortunately, Mongoose sees this and is keeping the non-prepaints for SST alive.