When was this the case? I seem to recall it being promoted as a multi-player game that could be played solo. The fact that it requires an internet connection kind of undermines the solo-first aspect that you are suggesting. While it is true that in early vanilla MP was garbage compared to SP, Blizzard immediately recognized that as a MASSIVE problem, because they wanted to promote MP over SP.
Regarding solo players not wanting to be forced into grouping, your solution would PREVENT them from grouping if they ever decide that they want to. To me, that is an unacceptable solution.