Model building isn't going away.
The things that excite young people so much that they want to build models of them are what's changing.
Fighter jets and spaceships and tanks and pirate ships and fictional and historical figures? Still evergreen. Still going strong. Even growing in some sectors. Add to that wargaming miniatures and Gunpla and you have a hobby that's thriving on the whole.
Car modeling? Oh boy... Car modeling...
Car modeling globally isn't actually doing too badly. New companies like Belkits and Beemax have found comfortable niches producing quality racing car kits. Aoshima has settled into supercars while Fujimi keeps the more "pedestrian" cars coming and Hasegawa does the historic thing. Building an identifiable piece of racing history, particularly one that these young builders have created a rapport with often by "driving" it in PC sims or games like Forza, Gran Turismo, or Grid, makes these kits as dependable a seller as your usual array of Tigers and Spitfires.
Muscle car, custom, and hot rod kits like those that AMT has focused on since the early 60's? Their audience is a little bit more finite. Ala Kart is a massively significant car in automotive history, but the number of people who have actively interacted with Ala Kart drops each year, and so the number of people who want a representation of it in their daily lives drops. Gradually, with the exception of a few "evergreen" examples, most every bit of that generation of automobilia will become niche. Once they're in that niche, it will be the role of short run, higher end model manufacturers to serve that significantly smaller market (in the model aircraft world, look only as far as the sheer number of different short run companies there are in 1/72nd scale)
If you don't believe me, think about the last time that someone mentioned Pierce-Arrow or Winton to you.