I feel like the sandbox is finally at a place where the Magnum is finally effective enough to be a starting weapon and the BR is finally strong enough to be a pickup on Arena and BtB maps.
Warzone has so many vehicles, so much infantry and so many power weapons with maps so large and with so much cover that the use of BRs and DMRs are necessary to play the game properly. This isn't the case for Big Team, where there are potentially four to six vehicles and 8 enemies at most at any point in time, each of which has the same exact equipment that you do or an equal opportunity to have gotten more powerful equipment.
BR starts actively hinder maps movement and vehicle use in Big Team. They have served their purpose of acting as a crutch to compensate for how utterly weak the starting weapons of the previous Halos were.