All this does will make people either
A: Solo more
B: Just not do the content
C: A few might just still find groups for it (but that won't be the majority)
D : A mix of the above
A and B being the most likely options. As seen in the past whenever Blizzard has tried anything similar to make grouping annoying or content too hard, most players just don't do it. Which maybe that is their intent, but it didn't do well in the past for them subscription number wise.
My Skyrim, Fallout 4, Starbound and WoW + other game mods at MODDB: