Am surprised there is no progression system in this game at all when it was one of the best parts about Reach. The Credit system was addictive and rewarding; anything you did gave you cR and that let you buy a slew of things, from armor and attachments to dumb stuff like effects and firefight voices. There were also the daily and weekly challenges which - while not without their own problems like forcing specific playstyles - gave players a reason to go through diferent aspects of the game each day. I looked forward to Firefight Thursdays, and I know many people who ran full parties just to chase after Inheritor. And that lasted over 2 years
Halo 4's "SR" system barely lasted a few months and now they've done away with it entirely. So now there's little incentive to go through the Campaign again, and with the way the lobbies are laid out currently, not much reason to play Arena for a skill rank either. Warzone is the only mode with a tangible payoff by way of using and unlocking REQs, but I find the progression tied to that is way less satisfying than what we had before.
For all the efforts to modernize Halo, they didn't make much progress in their investment systems.