Boooo, op. Boooo.
Seriously, mobility creep is about the only reason that many older champions see nearly zero play. Of the less mobile champions played most recently, the majority are insanely hard to kill tanks with heavy cc that have come to dominate the current meta (due largely to the addition of Righteous Glory, which, surprise, gives them the mobility they need to be relevant again) and the rest are immobile champions with the heavy dps needed to kill those same tanks.
It's actually funny because mobility creep is the very cause of the current meta, as it was the meta before that. It's not considered a heavily domineering design flaw for nothing, and even Riot has admitted to it directly. The challenge was, what to do about it.
Thing is, League is currently all about its competitive scene, because that's where the money and publicity are at. And what draws spectators to League? Flashy plays. People love flashy plays, and flashy plays require mobility. There's nothing flashy about smart positioning or out-strategizing your opponents. Overloaded kits with tons of mobility are exciting to watch. Doesn't have to be fun to play as or against, those guys are getting paid. It just has to be fun to watch.