Wait is that me in this screenshot?
One thing that isn't implemented yet is challenges - once those come online it should offer another avenue to earn XP / speed things up a bit. Overall I'm told that the total grind time is less than the OG version of Reach (but remember Luke Smith acknowledged he may have messed up a decimal point back then and inadvertently made it too grindy). As for campaign/offline - I believe it's unfortunately a necessity to limit progression to matchmade / online authenticated matches only, otherwise it'd be pointless and folks would just hack and manipulate it. (arguably not that it _really_ matters, it's not like it's a 'premium battlepass') Lastly, one of their main goals for this new system was building something that could span all the titles in the MCC with one unified solution. Not totally sure how it will materialize but when CE launches the same overall system will be used for that season, and so on.
I'm sure the team is continuing to monitor usage and earn rates and of course we're already listening to feedback so there's a good chance we'll see some degree of further tweaks over time.