The actual problem with RTS games from a longevity standpoint are actually the same problems fighting games have:
1. It's a 1v1 game, so you either are playing alone, or if you want to play with a friend, you have to fight each other. That's fun for awhile, but typically one person will be better, and the other person will get tired of losing.
2. They are really hard to be good at. On paper, RTS games are really hard because you are managing a lot of things at once (simplified by saying macro and micro game), and you need to be good at everything. The difficulty of a game is magnified when it's 1v1 because you only have yourself to rely on. So if you are having an off day, you are going to keep losing.
The reason MOBA games are popular is because they fix these issues by being a team game (very easy to play with a group of friends, and if you are not good or having a bad day, you can lean on your teammates to pick up the slack), and reducing your general focus to managing 1 hero (obviously, MOBAs have many layers and systems that make them more complicated, but at anytime you just need to focus on one hero).