One thing I haven’t seen discussed, when it comes to solo players is that most don’t play as often, or for as long a time. Many stop grinding a while after they complete the campaign.
I’m certain some of the impetus for pushing grouping is to slowly integrate them into that mindset, hoping they’ll get seasonal passes to play with their friends. Of course, a decision like that would work better in a game like WoW, where there is a guaranteed monthly fee.
Diablo does not have one, and they already have people’s money. Therefore, they shouldn’t care if a solo player burns out on the game and stops playing.