With the 10th anniversary, you generally want to do something big for the game and Senna being the next champ is perfect for that. With Senna being next...
* You conclude/progress one of the longest running rivalries in League's history (Lucian vs. Thresh)
* You pull one of the great lore figures who still isn't a champion.
* You silence people who think lore is still JUST retcons and rewrites.
* You add a major champion who will change the story of the world (or at least the shadow isles) in a major way.
* You might even _kill_ a god damn champion. (I don't think Lucian or Thresh are walking out of this one without at least one scar).
Not sure if this was timed on purpose or just a happy accident but it all lines up in an immensely satisfying way.