That is completely true.
It's also true that Overwatch will, sooner or later, lose its player base (and I don't mean to knock it! The SO and I both play). At some point Blizzard is going to need to scale back on promotion, events, and prizes. And what's left of the Overwatch League at that point? An e-sports league dedicated to a single game is ephemeral; one of the biggest advantages major league sports has is history. Narratives build around around teams, cities, players, and beyond. The Cubs' World Series win this year is one of the most amazing sports stories I'll ever see in my lifetime. I think baseball is generally boring as hell, but I can still get caught up in a great story.
E-sports leagues dedicated to a single game are not going to have the chance to to build that history and the stories that come with it. A league built with the ebb and flow of video game popularity in mind could support a broad number of games (maybe with developers directly backing) while being able to pivot its biggest events and promotions as time goes on. Teams* and sponsors, which drive leagues, can follow along. Spectators get something they can rely on not just for the next five years but for the next twenty or fifty, building the loyalty and history that ultimately matter.
I think I'm rambling now, so TL;DR if we want e-sports as a whole to prosper we can't depend on leagues that put all their eggs in one basket.
*when I say "teams" here I'm thinking of Kluwe's suggestion of an umbrella team that has multiple groups dedicated to specific games