And well,the champs whose story depended on the IoW could be adjusted to fit the new lore without changing their background that much.
A lot of the stories were only so good because of the league.
Jax just being king of some local tournament, Kayle asking help from just a random mage and Morgana some (probably other) random mage, Cho'Gath corrupting some random mages, those stories are not as strong.
They'll just get a different story. That's fine. the new champions would've had a different story for the old world too.
They might just get a new character too, but that isn't necessarily the case. When it is though, I've an issue with it. (But I think that's outside the scope of this discussion?)
The thing is that overall it simply started to feel repetitive.
What you describe is exactly the premise feeling limited. It felt like it was limiting the stories Riot was telling.
It didn't have to. They could've not used the IoW as conclusion of every single background story they made. But they did, so it felt limiting.
But they don't tell a single story.
There are over a hundred champions in the game and the storys mainly focus on the champions,not the IoW.
All the old stories were working towards them being in the IoW and being involved in the grand IoW storyline.
Now they're all separate stories, connected by nothing but being in the same world. Even that is at times questionable, as Riot has insisted on making factions of the world be isolated. (A bilgerat, inhabitant of the most infamous port, might've not heard of Demacia?)
It still boils down to the premise feeling limiting and there were non-champion characters powerful enough to be main characters. Those're the two arguments the retcon was based on.