The League did seal all of the exits from Fiddlestick's chamber, but you may be right that he could leave if he wanted to. I'm pretty sure "fighting with magic...and getting surprised by the raw amount of fire" is a description of the accidental creation of a treant. Maokai was certainly created by the actions of summoners, and it wasn't intentional.
Fiddles was ripped from another world. While anyone who approached him was consumed by fear and crows, those that kept their distance were unharmed. Yes, the league set up barricades, but honestly he never moved from his initial summoned spot. They prayed they could hold him, but were grateful he never challenged their setups.
Maokai, if you bothered to read the JoJ back before it was removed, was certainly birthed by overuse of magic, but it never specified that summoning magic was solely to blame. Rather, it was insinuated that the raw amount of magic, from the rune wars to the summoners to the champions themselves, was to blame. Also, the reason it erupted on TT and not on SR was largely attributed to the fact that Zaun had been dumping toxic waste there.
Thing is, their logic of "SUMMONERS OP" was pretty poorly explained, and quite honestly doesn't hold up for the entire concept of the league getting removed. You could just as easily removed the summoners, and simply had the champions represent their chosen side of every issue for the IoW. Even the one-offs like Nami could temporarily side with a certain cause for their own gain, or like cho for their own amusement. Certain champions would still need rewriting, but alot of those simply need it regardless.
As for why would anything else matter, I could make the same statement for the current lore. Nothing ever really happens, outside of another "A CHALLENGER APPROACHES" scenario. Even then, it's simply them appearing, and regardless of how you look at it, still effectively doesn't make anything happen.
Azir raises his kingdom and... nothing. Noxus doesn't care. Demacia doesn't care. No one but Azir cares. Xerath gets free, and... nothing. He doesn't even add anything to the landscape. The game is called "League of Legends". One would assume that everything in said universe would revolve around said league, and it's inhabitants, and how they interact with one another.
Now, there is hardly any real interaction. I mean, Azir saved Sivir's life and... nothing changes. She goes right back to her thing and Azir doesn't even care. I don't know about you, but if I brought someone back from the edge, I would at the very least feel responsible for them to an extent. If my life got saved, I would feel some slight sense of owing them for my life. But even that fails to ever cross into lore.
In the old league, the little interactions between champions stood side by side next to the major events. For example, blitz effectively made KatxGaren cannon with his matchmaker. This was equally important as all of Kalamanda, as it was Blitzcrank trying to understand love. And this was something that stood out, even if it was just something that blitz used the one time, it had interesting side effects between other champions and even a certain summoner.
At the end of the day, the current lore fails to prove its worth, as well as fails to prove why the old lore needed to go. Nothing the new lore has done would have been invalidated by the existance of an IoW. While one could argue the semantics on Summoner Power level, you would only need to change very few things and keep the league to make it work (specifically, they would need to fix other worldly champions like Kayle/Morg, Old Nasus, even Nocturne to an extent.)