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I'm not talking about having companions (npc or player)...but one character being able to solo 100% of everything. Now...hear me out...
Many MMOs, especially themeparks...they in a way...very much mislead you. You can spen levels 1 to 85 (as an example) being able to do EVERYTHING solo. Suddenly you hit max level...and the game had the bait...then it switched...suddenly there is very little content you can solo. You can only do raids if you are in a group, or if the raids become out of date (and that takes a while)...only very few (if any) classes are able to solo (when, while leveling, every class can solo everything).
MMOs are already on the path of becoming quick to play, cheap and more solo oriented. Log on, do raid/dungeon finder...play for 15-30 min and accomplish a lot. Not relying on spamming chat "lfg" "lfg"...15 minutes later... "lfg". And many MMOs are very quickly relying on the player WANTING to group, and not feeling FORCED to group. WoW went far down this path, but to do new raids...still need a group and (last I saw), new raids aren't part of the raid finder. SWTOR was on the way to this path, but found voice acting and adding to the story was much more expensive than adding raids that forced you to group.
But you can still group up while leveling in WoW and themeparks, doing dungeons and what not (that are still solable with certain classes)...if you want to...not if you are forced to.