This is a perfectly valid point. Sony right now is focusing on their strengths, which are story driven single player games, much like Microsoft is focusing on their strengths, multiplayer focused games. Yes the lack of a big multiplayer game is a gap in Sony's lineup, but the thing is, there's so many huge third party mutliplayer games, significantly more than there were when Halo and Gears were at the peak of their popularity
Unless someone is super passionate about Halo and/or Gears, there's really no real reason to chose one console over the other. Conversely, while Xbox has had a few single player focused exclusives like Quantum Break, that just can't compare to Bloodborne, UC4, Horizon, etc. And I liked QB quite a bit
Another thing to consider is the buying habits of multiplayer fans vs single player fans. The average gamer who plays predominantly multiplayer games will usually chose one or two games a year and they keep them occupied until the next year. Again, unless it truly means something to someone that one of these games is Halo and/or Gears, it doesn't really matter which console they chose, so they follow their friends. Single player fans tend to buy more games each year, as once they beat the game they are currently playing they go buy the next release. And the undeniable truth is that there's more single player games available on PS4
My point with this is, while Sony doesn't have a big first party multiplayer game, they really don't need to. There's plenty of big third party ones on the platform, just because they aren't exclusive doesn't lessen their value to owners/potential owners of the console. As for single player fans, it's a far better bet to chose the console that in 2017 alone has seen the release of Gravity Rush 2, Yakuza 0, Nioh, Horizon Zero Dawn, Nier Automata, and Persona 5 along with Resident Evil 7 and Mass Effect Andromeda, than the console that only has Resident Evil 7 and Mass Effect Andromeda
That is also why Microsoft's single player efforts haven't found much success this gen. People who are passionate about those games are on PS4. People who are passionate about multiplayer games are pretty split between both consoles