The trick to a modern succesful RTS is NOT a good singleplayer.

The trick is a good EVEYTHING and that's why so many indie devs struggle with it as making the next big stuff needs tol many resources, and there's not this kind of budget anymore for an RTS.

An RTS needs a ton of stuff to be succesful, stable and to ladt a long time: A good campaign, a good & balanced multiplayer (with ranked), coop or "party/chill" modes and editor or tool to create maps & new modes. And well good graphics/gameplay/mechanics/game-feel.

If you look at the great ones in the genre. They all had that. They all had weird modes you could play with friend in custom, a great campaign, multiplayer, etc...etc...

Nowadays most RTS only focus on a single thing. The problem is not the focus on multiplayer a focus on single might sell a bit more but games dry very fast after few months.

The only oncoming game right now that seems to be aiming at "everything" is Dawn of War 4, most of the others always have the focus on only a single thing.