I think you need to make a way to do both. As in: The major factor WoW had over EQ was for players to be able to make character progress without relying on player groups.
In WoW the pendulum swung way to to much into "the best way to progress is by soloing until you are max level, and the only social interaction is by raid/M+ for character progression." Blizzard basically cut out a large portion of the leveling process out, which they specifically designed for the solo part.
What they should've done:
- make single player quests reward less XP (there's tons of them by now, so players won't run out of quests any time soon)
- make group quests and instances reward more XP
- give an XP buff for instances with parties formed outside the automated matchmaking LFD
People would still be able to progress, but on top they have an incentive to look for players on your server to play with.