I used to do primarily military subjects until the colors got bland and the variety dwindled down to just a few types. Airliners were so common and most every airport had a viewing deck or place to photograph. I think this availability is what brought us such high quality images from a few decades ago. The photographer could pick the best of conditions. Military photography could be hit or miss depending on the access and weather.
Not knowing where my interests will lie in the future, I photograph the whole spectrum of aviation now.
Cottage Grove, MN