>>50811694>>50812935It's worth noting that Oak is allowed to enter the Hall of Fame, which according to future games means he beat the League himself once.
Based on that, and his background with Agatha, the games seem to imply Oak was in fact the previous champion and retired years or even decades prior, possibly a remnant of his planned boss fight.
As to why there was no active champion in the meantime, the best explanation I can think of is that there were no League worthy trainers for a long time, not even to fill the Elite 4 until Will and Karen arrived and Koga got promoted. So instead of promoting Lance, they prioritized keeping the E4 intact and left the official champion title vacant. This all makes sense when you consider the next logical choice for the E4, Giovanni, was largely absent.
