At about a minute in, listen to what Jason Jones says:

How do you keep a player going for 50 or 100 hours over some number of months?

This never struck me until now. That sounds like a much more traditional and less grindy game… it's just very curious how the plan changed.

I think players started grinding D1 all on their own, and having seen it happen, Activision decided not to let Bungie forget about it.