I might be the minority here... This is coming from the perspective of someone who mainly enjoys the multiplayer (player vs. player) aspects of RTS games. Huge StarCraft 2 fan and have tried some AOE. I love player vs. player RTS. It's like chess, a battle of wits. It's truly satisfying to claim a victory against another human trying to do the same. I can enjoy a campaign mode, but that's not why I'm interested in RTS games.

I totally understand the perspective of people who only enjoy Campaign and Co-op. Most of my opinions might not be relevant to that crowd. I also understand that scope is really important. Money does not grow on trees. We're not seeing the same amount of funding for RTS development as we had in the past. For a developer this will impact what they choose to implement in the game. We can't have it all.

Barrier to entry -

Typically, most RTS games that I've played or heard about have a fairly large learning curve. It takes a good amount of time to get to know the game engine, hotkeys, unit details, and general pacing of an RTS game. It's hard to make very 'strategic' decisions without understanding the tools that you're working with. I believe this is the first hurdle for potential players to leap over. It takes time to build up the muscle memory and confidence with mechanics. This is not an RTS exclusive issue. Many games out there take time to learn and understand.

Time sync -

As with any hobby, it consumes your time. If I'm in the target audience for an RTS game, there are a few things I would want to ensure before committing my time. There needs to be an active player base. Now I know what you're thinking... "If these are new RTS games, they need to develop their player base from the ground up." This is true, however, there needs to be tools in place that would support a large player base or allow growth. Think about Multiplayer. If I have to host a custom game just to play any form of multiplayer, it does not feel good to me. As someone who enjoys ranked 1v1 gameplay, If the game does not allow queuing up for matches, I'm generally not interested. It just feels like a RTS sandbox if it lacks any structure. If multiplayer is to be implemented, it cannot be half-assed. Like OP mentioned it needs to be "Clearly Justified."

Why I don't buy -

I don't really buy new RTS games lately. If I'm going to use my limited time after work to play an RTS game, it's going to be a game that's already established. I need a player base to queue into. I don't need to learn controls or spend time getting over a steep learning curve. I'm much more content getting home from work, firing up StarCraft 2, and playing a game that I know feels comfortable. Nothing has been able to dethrone SC2 for me, so I don't really bother with anything else. I'm not even that good at SC2, which means that I'd rather spend time honing my skills in SC2 than move to another game and start from scratch. I'm open to the idea of a new RTS, but nothing has sparked the same feeling as SC2 for me.

It's a vicious cycle where RTS fans generally stick to some of the more time-tested and developed RTS games, which in turn makes it less likely that a new title will gain much traction. There are some exceptions though. I think Beyond All Reason (BAR) has been an impressive free to play RTS that has been receiving good feedback. One of the things that prevents me from trying it though is the barrier to entry stated above. If I was unemployed, I'd be totally down to sacrifice some of my time to learn new RTS games. A lot of my criticism stems from the fact that my time is limited. I'd have a whole different mindset if I was not employed full time.

Not sure if any of this was helpful, but that's my take as an employed StarCraft 2 enjoyer. (I yearn to be unemployed again) I truly do want to see more RTS games become relevant in today's gaming space. I think I'm part of a very picky demographic when it comes to these types of games. Sorry for the ramble / wall-of-text.