I went in the opposite direction. My first few models were aircraft,,,,,,then I started getting tanks, ships, cars, semi-trucks, engines, dinosaurs. I cut down to just ships and aircraft a few years ago, and then down to just aircraft (with a few ships)
I was an "any air force, any era" kind of guy,,,,,,,,and had to cut down to a more narrow grouping to simplify things.
I do find myself letting "off topic" aircraft creep back into the collection, though,,,,,,,,there is something about a Tiger Moth, or Typhoon, or even a US F-89 that calls to me at the shows and shops. And I have absolutely no idea why I have a Japanese Betty in the stash, other than Anne saw it cheap at a model contest.
Rex