If you ever talk to ppl about their favorite Halo moments 9 out of 10 times they are going to bring up moments they played in coop and much of that will be couch coop stories. Even though Halo revolves around a single Hero, Masterchief, it’s always felt bigger than that. Many ppl even create back stories for their Masterchief sidekicks.
For me, anticipation of every Halo sequel has been centered around expansion of coop. Halo Reach and Halo 4 made me the most hopeful of things to come, but the last two Halos have severely cut into the coop experience and hope.
There are a ton of single player games out there, but other than sports games there is almost nothing you can play socially when friends come to your home. Coop reignites a game you have already played through and brings new excitement, making the same game new again.
Every game that has pvp and pve always have a divided community as to which is their favorite and which side gets the developer love. Developing these two sides can be difficult in RPG or MMO style games, which is why Halo can succeed where they fail since it is neither. Balancing is much easier. The money is almost always on the pve side of gaming because pvp can be more toxic.
If coop is being postponed in order to make it more epic then hats off to you and keep at it, but if you think Halo is going to survive by having just a great pvp mode then I think you are mistaken. I think you have to get pvp functioning, but the most resources needs to be in creating more expansive and functioning coop.