It would be one thing if you were going to retrofit the client into some kind of Launcher for all the games Riot intended on making (even though it's held together with duct tape), but now we've seen that this isn't your intention.
TFT is not a League of Legends gamemode.
It's an entirely different game from League, with its own audience.
The things that draw people to League are completely different from those that draw people to TFT. The audiences do overlap, but it's not much.
If they don't want to play League, TFT players shouldn't be forced to keep League's (comparatively large) file on their PC.
If they don't want to play TFT, League players shouldn't be forced to not only keep TFT's file (however small it is) on their PC, but also shouldn't be forced to watch League modes be *removed* with the devs pointing at TFT as some dubious defense.