You mean becoming a massive and highly profitable company with only one game as your sole source of revenue is a bad thing?
Seriously though, there are too many chefs meddling with the soup. It's crazy how they aren't putting any resources at all into another title.
For comparison, and with a note that they have more staff, let's compare Riot's game to another studio with a long-running and continually supported game...Blizzard.
In August it will be League of Legend's official ten year anniversary. It has had several game modes, one overhaul of the main map, and regular cosmetic and gameplay updates.
Same can be said of World of Warcraft. Except with the 5 expansions over the past decade it has added real estate, races, classes, core mechanics, as well as regular updates.
While Blizzard also released Diablo 3 and it's expansion.
StarCraft 2 and it's two expansions.
Hearthstone.
Heroes of the Storm.
Overwatch.
Riot needs to make new stuff,and not in LoL.