Of course it's utterly unfair to say it has no competition. It had plenty.
However, it's fair to say that at the time, shooters weren't all on the same page. Halo was one of the first games that really got online play right on console, Halo, and Call of Duty, and these were the most popular games at the time.
That gave it a big boost, it was the multiplayer shooter to play when we moved from the original, to the XBOX 360, and it'd already established itself as that. Other games were still getting their feet with their network infastructure, Battlefield was very messy on console before Bad Company in 2008.
Halo gets a big boost in sales on its platform because people purchase the platform to paly it. Meaning it has a good chance of outmatching the sales of its competitors, even as its universal popularity dwindles, because exclusives sell the system, and Halo didn't have the power to sell the XBOX ONE, like it and Gears of War did in 2007.
That isn't to say the new game is bad, not at all, but game itself has less appeal, while the game wasn't without competition in 2007, the competition it has now, feels much closer to being on the same page. Halo just isn't one of the few titles doing things right anymore, so the fact that it is, matters less, giving it less influence in selling the system.
In 2007 if people wanted a strong multiplayer fps, then what did they buy? It was Halo, or Call of Duty. Sure, there were lots of other shooters, but if you narrow the competition down games with a good multiplayer infastructure and community, only these two spring to mind. Today we have Rainbow Six, Battlefield, Destiny, Call of Duty, Starwars Battlefront, Titanfall, all of these are strong multiplayer franchises offering a similar set of features.
it's utterly unfair to say Halo was popular because it lacked competition, but Halo 5 is less influential because it has more matched competition. Sure, you could argue for days about the merits of Halo 5 compared to the less thoughtful shooters like Titanfall or Call of Duty, but the average consumer doesn't see that. They see a franchise with a smaller following, a franchise getting lower critical scores, on a platform that less of their friends are playing on. I'm certain Halo 5 is a great game, but it doesn't matter, it doesn't stand out as much in 2017 as it did in 2007, and the performance and reputation of the XBOX brand helped cripple it.