This game hardly aiming to steal audience from RTS, MOBAs, survivals, fightings and stealth + RTS and MOBAs have scrubs and baddies as their main audience.
http://www.leagueofgraphs.com/rankings/rank-distribution - observer Rank distribution in LoL.
You see how little people overall get to Diamond and Platinum? And I am not even talking casual mode that is not shown here.
No every game needs fanatical dedication, sometimes game just needs to be fun to play.
And both RTS and MOBAs are more or less at their "funniest" stage for me when you just messing around as scrub with other scrubs - no rigid buildorders, no rigid meta picks, no raging on shit going south, cause you just messing around and no hard feelings, you learn new things and thats why loss is not as harsh. But once it stops being game and becomes competition I lose interets very rapidly.
I play games to have fun, not to compete with others - and alot of people do that too. Because I for once have real life, real problems and enough headbutting with people IRL, so its not welcome in games. I also have stable income and money that I am willing to spend on recretional activities like games and movies - if they are worth my time and leave a feeling of satisfaction in the end.
And definition of "casual coop shooter" is vague at best. I know only one game that is semi-occupies that spot and feels great doing so - again - Warframe.
I know no game of same caliber who comes even close to Warframe in terms of how much stuff you get to do in it for free. And not only Warframe has all that stuff - its still keep on giving - development is slow but steady and if you pick Warframe now as new player - you will have few years before you will finish every possible activity in game and max all equipment.
And Warframe did that because how inclusive game is and how you can pretty much be completely freeloading and get every game-play reliant stuff for free - if you willing to dedicate time, or you can buy everything with plat if you cant wait, or anything inbetween.
What else can be considered casual coop shooter? Killing Floor 2?
When you say RTS and MOBAs - I guess what you mean is Starcraft 1-2, DOTA and LoL. Because no other RTS is currently has any sort of relevance - its alot of them on the market, but they dont have millions or even just tens of thouthand people playing them.
Same for MOBAs - there are alot of them yes, but aside of LoL, Dota and probably Heroes of The Storm - all of them irrelevant right now. Strife, HoN, Dawngate (RIP) - where are they now? I guess Vanglory is doing ok - but its niche mobile title.
"Bloodline champions" died out because of being too hardcore and thus being unable to sustain decent playerbase + breakge of promise to not give stats edge to anybody for sake of giving people progression. Newbs were slaughetred by vets who not only had superior game knowledge but also superior stats on their characters.
Current incarnation of BC - "Battlerites" - is doing better but has same problem - game requres alot of dedication, something that alot of people just wont do - why waste hundreds of hours on another PvP game? But atleast they are not repeating same mistake with giving Vets more power.
Fighting games are niche and again most "relevant" of them run on well known franchises - Street Fighter 5(!), Mortal Combat 10(!!), Gulty Gear Whatever, King of fighters 14(!!!). Injustice is "kinda" new but its basicly Mortal Combat with DC crew. Again lots of dedication needed to play on top lvl that not alot of people will be willing to do.
"Raiders of Broken Planet" are trying to do what Battleborn, Evolve failed to do. Be many things at once - its coop, its PvE, its asymmetrical PvP (symmetrical in case of Battleborn), its B2P with F2P mechanics of skins and tickets, its story driven, but relies on multiple repetition to get items you need. Fail a single aspect and game overall will crumble.
Games like "Overwatch", "Starcraft", "Street fighter", "Dota", "LoL", "Witcher", "Nier Autromata" - share one simple thing - they are are not trying to be many things at once.
They do one thing great. And they also inclusive while doing so - some by design (difficulty setting in single player games), others by virtue of having HUGE playerbase and thus being able to mach pros with pros and scrubs with scrubs without turning matchmaking into slogfest.
But if you dont have virtue instantly huge playerbase (cause you are not Blizzard and thats your first online game release ever on a very scetchy busines model of episodic releases) hardcore aspect of the game should not be main concern. Diehard fans who just like whole idea will play no matter what, even if expirience will be subpar - but other players wont be so kind. Again its alot of different games around - and if one isnt fun - they will find one that is.
But ofc, I am just a scrub who wants my game to be fun, instead of constant struggle. And want alot of different people playing alongside, even if sometimes I will have to carry them or give advice.