Ask the thousands of players at Warthunder and World of tanks how much of a niche game these are.
Sure. However look at this game. Now look at War Thunder. Now look at Counterstrike/COD/Halo/generic FPS 347.
Forgetting about the fact that they're in vehicles, which game is the actual gameplay of War Thunder closer to? I'd say Counterstrike or any of the other FPS's out there.
Small groups, even number teams, 20-30 minute objectives, get a reward at the end of it.
Open world MMO PvP is most definitely a niche market, and draws orders of magnitude fewer people in to play it. The closest thing to this game out there in gameplay is actually Planetside 2, and their numbers are again orders of magnitude smaller than games like WT and the FPS's.
I wish it were otherwise, but it's plain as day that not very many people want this kind of game play.
Wiley.