Yeah, that is the one flaw with esports. A game won't stay mainstream forever, and publishers are going to abandon it after awhile and move on to other things.
But the esport scene is also driven by the improvement in technology as it continues to involve itself with the people of society. 15 years ago, no one expected people to have pocket computers and now practically everyone does. The industry's success is also built off of how fast tech improves (console and pc etc) and how accessible it is to everyone. Part of the problem right now is accessibility, as hardware requirements inhibit people from playing games they would want to play. If it is more accessible, it means more players, which incentivizes publishers to push out more content or invest more into their game.
Granted, diminishing marginal utility actually applies (more frequently) to esports unlike physical sports, so something needs to change in order for the industry to even make it to the stage Sports currently have.