Eh, I don't doubt Cliffy's assessment, as Aardvark already pointed out, the level of content between a AAA campaign and developing a multiplayer mode is leagues different, and its understandable that developers that want to make a multiplayer shooter would want to save money by cutting a feature that most players of online shooters like CoD and Battlefield don't bother playing.
The problem comes in when it becomes hard to see how those multiplayer only games give us any more than games with multiplayer mode and campaigns. Titanfall launched with 3 mechs, 0 customization at launch, a small number of weapons and abilities, and unimpressive post-launch support. Evolve launched at full price and then expected us to pay even more for more monsters and customization. Battlefront launched with a disappointing number of maps, samey weapons, lame customization, and frankly just far too few gameplay modes.
When you cut the campaign out, you need to show us what we are getting in exchange, it seems like shooters that cut their singleplayer out give us little in exchange for apparently freeing up such a massive amount of its budget. A multiplayer game with 75% more budget for its multiplayer mode should at the very least come with more maps than the average Battlefield game, more customization than CoD, and either the amount of gameplay modes or even outright rule customization like the Halo games. If you can't even match the variety offered by series that have content rich multiplayer AND a single-player campaign, then why should I waste money on your game when I can apparently get equivalent multiplayer content and a single player campaign for the same price.
That's not to say these multiplayer only games are bad, I quite like Titanfall, and Rainbow 6: siege was fun when I played it at a friend's house, even Battlefront was fun for a time, but all those games lost my interest quickly as it just didn't feel like I was getting enough variety to keep playing for months on end, and a multiplayer game only lasts as long as its player base can stay actively playing every day.
SO yeah, don't tack on a single-player if you want to make a multiplayer only game, I have no issue with that, but show me what I'm getting to make up for that, if you're showing me the same level or even less content than I would get from the big name shooters out there like Halo, Battlefield, and CoD, where I can get multiplayer and singleplayer for the same price you want for multiplayer only, don't expect everyone to buy into your game, no matter how hard you try to justify the lack of singleplayer.