One of the broader issues game developers are running into (certainly Firaxis is now running into it with Civ VI and we ran into it with Ashes of the Singularity) is that nowadays, your "new" game is competing against every game that has ever been made.
In the good old days of 2010, my game only had to compete against the 38 other SCUs on Walmart's shelf. That was the entire market.
Now, my $59.99 "new" game has to justify itself against fully realized (but "old") games from two years ago that are now going for $15 on Steam sales.
When Ashes of the Singularity came out, it was a $49.99 game. And many players who bought it would say "I like it..but...I got Supreme Commander: Forged Alliance for $10"
Being able to say "Yea, but we support more units and higher resolutions and blah blah" is a tough cell if you're asking for, in effect, a $40 premium.
When Sins of a Solar Empire shipped, it only cost $800,000 to make. So obviously it didn't have a huge amount of content or polish yet (see Tom's "Sins of Sins of a Solar Empire"). But it didn't matter because if you wanted to play a space RTS, it was, literally, the only game in town.
Since Sins came out, there have been other space RTS games that have struggled. Not because they were terrible but because, at the end of the day, you can get Sins of a Solar Empire inexpensively.
This is an issue that isn't specific to the RTS market (look at the Steam user reviews for Civ VI -- it has to compete with a fully matured Civ V that is half the price). But it affects RTS games strongly since they tend to age better.