Tobold proposes that players prefer to solo because the exp is better, i.e. that players choosing not to group while leveling, sometimes to the detriment of their own gaming experience, is an incentive issue which can be solved by throwing more exp at the group players. Why haven't studios tried this?
The group/solo decision is not purely an incentive problem
First of all, the suggestion that players will group more if presented with better rewards is, at best, an oversimplification of the issue. A player who cannot commit the time needed to find a group and complete the group content will not be able to change that reality simply because someone has doubled the rate of exp gain per time for dungeon content. (In WoW, the bare minimum time to clear a 5-man heroic dungeon is somewhere between 30-60 minutes, depending on the dungeon, but that time can easily double in a group that needs to practice the fights, and triple if you just can't find a healer.)
This added overhead to attempting and completing group content can be improved via accessibility - this was what made Warhammer's Public Quest feature sound so exciting - but it cannot be entirely eliminated. One point that Tobold DID catch is that we can't be talking about SLIGHTLY more incentive to group. Because of the time investment required to find a group, players probably aren't going to bother for a small exp boost. You need to be offering LARGE amounts of exp and gear before it is worth the hassle.
Scarcity of Content
Looking beyond that, for the sake of argument, Tobold is effectively proposing that a game be designed with two separate exp curves - one for solo players and one for groups. This is not a bad idea in theory, but it runs into the 800 lb content gorilla in the room - developers simply cannot create content as quickly as players can clear it.
For the most part, the content in WoW is set up so that players solo all the way to the endgame and only then begin the group content. This setup causes various problems - players either do not want to make the switch or turn out not to be very good at their new group role for lack of practice, and, as Tobold complains, are not very community/group minded because they simply haven't had to be. Tobold suggests that the problem would get better if Blizzard simply went through and added a parallel set of content for groups that offered double the exp, but this plan cannot get past the gorilla.
The current setup in WoW is not accidental. Due to the time requirements for forming groups, the players who would be able to take Tobold up on this offer are the ones who are spending the most time online. After we're done adding the parallel content, effectively 1/3 of the game would be group content and 2/3 would be solo content. It simply does not make sense to take the most active players, who are going to run out of content the fastest, and have them completely skip 2/3 of the content in the game. It makes even less sense if you consider that Tobold's ultimate goal is to get the majority of the community participating in groups, at which point the majority of the community would be skipping the majority of the content.
Under Blizzard's current model, the players who can do small groups must first use up most of the solo content, the entry-level raiders must use up most of the small group content, and the elite raiders must complete the entry level raids. There is simply no way for Blizzard to produce equal amounts of all of this content in parallel. (Even if they did somehow manage it, players would promptly start poking holes in the incentive structures - when WoW launched, players overwhelmingly preferred to run the 5-man level 60 dungeons with 10 players, even though this drastically reduced the challenge of the content.)
A niche for scaling content
Tobold thinks there is a market niche for a game that maintains accessibility without sacrificing community. I think he's right, I just disagree that having a parallel solo content track is the way to accomplish this. The "every class must be able to solo" paradigm is not the only route to accessibility, it's just far and away the easiest to implement since it does not rely on any assumptions about the player having the appropriate number of willing group-mates of the appropriate class mix to beat the content.
The approach that might work would involve scaling content. Content would need to scale with the number of players available (from some minimum, perhaps 3-5 to start, going upwards to whatever limit the server can handle) to clear it and, more importantly, needs to scale with the ABILITIES of the group. The accessibility of current group content is hamstrung by the requirement that a specific proportion of the players take on specialized tanking and healing roles - activities the devs have not been able to make sufficiently interesting to attract a sufficient proportion of players to fill those roles. If dedicated healbots simply aren't enough fun, the devs could shift towards a game with no healers and some mix of self-heals (e.g. one class has a large instant self-heal on a cooldown, another class has a medium self-heal they can cast as needed, a third class has a small passive heal for dealing damage) to maintain the challenge level.
You would also need to offer options for shorter content, or some variation on the Warhammer "come whenever, stay as long as you can" public quest to maintain accessibility. And you would be critically dependent on having enough population to support the system (i.e. if a player was the only one online, they'd have literally nothing to do). Alternately, you could implement crafting with an EQ2-like separate exp bar and have that be your solo content.
Designing, implementing, and balancing such a system would be tons of work. However, the potential payoff would be huge. You would no longer to worry about parallel content - everyone playing the game would have access to all the content. You might lose some people who absolutely cannot play uninterrupted for more than 5 minutes at a time, but that's a given when you're talking about a niche product. And, most importantly, you wouldn't need to make enough content to go head to head with World of Warcraft. The WoW approach requires a volume of content production so large that even Blizzard can't keep up with it, and anyone else making the attempt will not have Blizzard's dollars to pay the development bills.
Anyway, the point being that sometimes the game, not the incentive curve, is the actual cause of the problem.