So Lucas was secretly making The One this whole time?
Obi-Wan was as powerful as he was because he was just that good. He was strong enough as a Padawan to take out a Sith Lord, and that was when there were thousands of other Jedi still alive. Yoda was pretty much always as strong as he was, no matter how many other Jedi there were, and we don't see either of those two getting sudden boosts in power as Order 66 is carried out. Obi-Wan, Yoda, and Luke were all as strong in the Force as they were because it was fate/the will of the Force/whatevs.
The Force is something shared by all living things--at least it was, until Lucas made it into something genetic that could be inherited by special bloodlines of Übermenschen. But even that aside, there are plenty of other Force traditions in the Galaxy (some of which have even been canonized in the movies and TV shows, like the Witches of Dathomir), whose numbers didn't seem to be culled by the Empire. The number of these other Force users doesn't seem to affect the strength of the Jedi or the Sith. I don't think the Conservation of Ninjutsu applies to Jedi (or at least it SHOULDN'T apply). Otherwise, ANY Jedi could have been the Chosen One, as there would be no 'fate'/'will of the Force', and ANY Jedi could have defeated Darth Vader.