I always felt that the existence of a "champion" in Red/Blue was supposed to be some kind of plot twist
While there are talks about the E4 late in the game there is never a single mention about a fight after that, as far as the story goes there is not an already existing champion. In fact Lance's unique room layout and defeat speech works as some kind of fake out ("Congratulations, you are now the pokemon league champion... Or you would have been (...)")
Blue's situation then raises more questions. Did Kanto really not have a champion before Blue? Lance's dialogue implies so, but then wouldn't Lance be the de facto champion. The whole thing makes it seem like the extra battle is out of the ordinary and under normal circumstances beating the E4 should be enough. Maybe there was a champion once but he got old and decided to fuck off. Speaking of Lance, how exactly did he become the champion in Gen 2? Did he just inherit the title as the highest ranked member of the E4 or did he decide to leave the E4 and challenge them like the player seeing the title was up for grabs after Red and Blue escaped their duties?
In any case my main point is that in the original game the "battle against the champion" was supposed to be some sort of surprise and the developers expected you to believe that you were done once finishing the E4. Naturally once the series exploded in popularity and peak poke mania was a thing there was no way to keep the surprise intact, much less after the release of following generations, where there are clear hints of a champion battle and the idea of an established champion was in place. Just thought it was curious to point out this yet another gen 1 oddity that often gets ignored.