Competitive multiplayer is what keeps RTS games alive. Last Stand and co-op are nice, but don't really matter in the long run.
The multiplayer in this game is awesome.
I suspect you have it backwards.
The co-op campaign and Last Stand almost certainly contributed a
ton to Dawn of War II's long legs. (Last Stand in particular is the only reason I bothered logging into the game at all after Retribution's launch.) The co-op mode in Starcraft II is getting loads of attention, with new commanders and maps being released every few months even this long after LotV's launch (remember, that game launched in November of
2015).
I would say that competitive multiplayer is
important, but the grand majority of any player base is not interested in competitive multiplayer. They'd much rather be able to just jump in and have a good time. 1v1 in particular is usually the least popular game mode for a variety of reasons, up to and including the incredibly high skill cap that goes above and beyond anything you'll find in a MOBA. At least in team modes you can put some of the blame on teammates for not carrying their weight (justified or not - it's a psychological thing), but in a game with as small a player base as this (and Relic games have
never had huge player bases), obviously that's not really tenable long-term.
I really think they'd be better-served by going the Starcraft II route and putting a lot of effort into a unique coop game mode with progression mechanics. Much lower barrier of entry, which translates to much greater mass appeal. It's pretty much how I'm playing the game already since I only ever play PvAI at this point, but obviously an actual coop game mode would have more interesting mechanics and more map variety than we have now.
Putting so much emphasis on PvP has, I suspect, contributed greatly to the genre's decline. MOBAs - hell, even
fighting games tend to have a lower barrier of entry than RTS games do. I don't think it's any surprise at all that folks just bounce off of the genre when it requires so much investment just to become competent at it, and even moreso when the player base is tiny and consists of only the most dedicated players. It's just not a good look.