Back in the 90s, some of the first digital cameras weren't much more than that[1], with Kodak and such making ones for specific SLRs. The part that I never knew about until finding this article was that a company even tried the direct "drop in roll of film replacement" approach in the early 2000s, but they couldn't get it off the ground.
[1] http://olivierduong.com/converting-film-cameras-to-digital-e...