Right now, the only thing we have is Summoner's Rift, ARAM (which is just a summoner's rfit cliff notes edition), Twisted Treeline and TFT.
If you kept the modes you had, you'd have... ahem:
One for All(Summoner's Rift)
One for All(Howling Abyss)
Hunt of the Blood Moon
Legend of the Poro King
Doom Bots of Doom
Black Market Brawlers
NEXUS MOTHER OF ZOE FATHER OF TEEMO BLITZ... ._.
You would have 19 different modes for players to have loads and loads of variety to play in.
Instead, you have 4, which since you're getting rid of Twisted Treeline, you'll soon have 3.
Also this isn't even including the Co-op game modes, like the Star Guardian event, the Totally not Guardians of the Galaxy event mode, that surprisingly fun Super Smash Bros Thresh Edition game, the all marksmen game mode and probably another mode that I missed.
Now am I expecting Riot to actually bring 19 different modes in? No.
But really, even if you picked the most popular from this list, we'd have at least 10 different modes to play.
Instead we only get Summoner's Rift, Summoner's Rift condensed edition, and the money grabbing RNG Little Legends mode.