Even in the old roleplaying games you could be a Jedi or what not unless your character was Force Sensitive. At that was 1987. So the idea that you have to be born with it to use the Force is not a new idea at all. Remember that Leia though she could never have Luke's powers, but was wrong simply because they were related.
If anyone could be a force users, than we'd see more of them around, rather than just a handful during the original films. Han Solo wouldn't have been a skeptic when we first see him.
By 1999 the concept that people who could use the Force were special already existed. At least one novel had attempted to have an Imperial force sensitive scanner that would read a person's aura to see not only how strongly they were attached to the Force, but also how much Light and Dark side they had in them. The midichlorians just gave Lucas a way to have the Republic test people's Force potential without resorting to auras. Mind you it is likely that everyone has midichlorians, it is just a matter of more amplifies the ability to hear the Force through them. They are not the Force, just a method of heading it.