Im an oldfag so I will try and answer.
They didnt evolve past a certain point. The logical thing to do would follow in the path of UT2004 and Tribes games. Focus on bigger maps and objectives that let shit players still contribute something, or just have fun zooming around.
Every arena shooter after a certain point was just trying to remake Quake 3 over and over.
And on top of this the arena developers listened to the siren voices of the 'hardcore' players who were never actually that numerous to begin with. When arena shooters were big the majority of people were dicking around in bigger games, using weird mutators, and consistently voting for the gimmicky non serious maps like longest yard. If you base your game on catering to the tiny amount of loud people who just want to play 1 vs 1 duel you will fail