I am 100% sure than in the short-medium period M+ing with friends only is fun. I know because I was into a friends guild from Vanilla to end of Cata and I remember those times in dungeons with a big smile on my face, even if M+ weren’t a thing back then.
But when you have not this luxury anymore and your spare time drastically diminish, well, you soon realize you just can’t waste half of your time searching for someone to do stuff with instead of doing stuff itself.
I have big hopes on D4 too, today I just received an invite for the end game beta (even if I didn’t even opt in because I don’t have time XD), if and when NDA will fall off I will be more than glad to share my thoughts.