We still don't have a good reason for why the league had to be closed.
The explanation that Riot has given us is just that the league was too confining, and that it was hard to expand on, and I am worried that this will be the only actual reason given for a few years.
Look at the evidence so far if you don't believe me. The new Azir and Xerath lores could have been written with the league still in place (neither died and was a long time ago), the time between lore events and champ releases take a while (which seems to be the only time we actually recieve new lores), and that the lore team is sounding like they won't actually have anything major like a champion dying off until every champion gets a new bio(which will take years at the current pace).
Meanwhile the Xerath retcon shows how don't really care how much the bios go against the champions established personality. Xerath's qoutes in game are almost all about how he wants more power and knowledge while his lore makes him sound like a "oh no what have I done" type of character.
During the Shurima event every lore piece of him showed Xer as a power hungry villain, yet Riot has already retcon'd that within a month. Now you may wave this off as just the lore team deciding on a better course of actions, but it still shows how the lore team doesn't care that much for the pre-established stories. What does that say about the rest of the old bios and themes of champions and how they might drastically change them?