I would like to start with congratulating with your attempt to communicate with the community, something which people had lost hope on.
First, in terms of Content, there is no campaign that is difficult to access or complete for all classes, so thumps up there. Thus, lets skip the 0.001% of the game experience from lvl 0-80. The same goes for any lower level dungeons except LOMM, CR and few others but even then, as long as you know the mechanics, you don't really have to be a hardcore players to complete them.
Then why do we find a very simply trial such as Tiamat with a huge failure rate? I believe the root of this cause is actually how you gate content behind the requirement of ILVL. A wiser choice would be to use a combination of factors i.e. primary stats and power. Many people have had put forward really good ideas on forums but most players that fail these dungeons are unequipped and expect to be carried. You cannot force them to learn the mechanics, but you can force them to be more prepared aka gear. It would be wise to focus on gating content behind a more realistic equation.
Jumping onto end end game content. I believe part of the reason why TOMM was disliked was because of it being a niche trial i.e. only for very hardcore gamers. Only you guys would know how much revenue TOMM brought but the matter of fact still remains that TOMM is actually a huge success. The reason i think it is disliked was because it introduced the holy trinity system into the game where players actually test their mettle.
As to why TOMM was a failure even among veterans. It was because the whole concept of the trial was gated behind few compulsory classes. I believe that the developer team failed to predict that such a trial would bring the pure holy trinity concept into the game for the first time in its history. In dungeons where hardcore veterans players clash, all that matters is skill. But when you bring in classes that are basically "who can spam faster" and still do crazy more dps, throws any sign of skill out of the window. You could have made TOMM a much bigger success had you balanced classes before its release and even to this day, you are completely ignoring the statistics.
I can with all honesty tell you that there is a minimum of 15% difference between a skill ceiling wizard and skill ceiling Arbiter, Warlock or Barb. That is the minimum and it could be more, but i do see people Exaggerate it to 40-60% but that might also be because Wizards are just so much more suited for TOMM than any other class.
Personally, i see neverwinter as a holy trinity game where skill matters and part of the reason why the new combat system was not favored. It is my opinion that even though more content should focus on the larger portion of population, the whole concept of upgrading gear (a core aspect of the game) demands there to be dungeons and rewards that scale with difficulty. In terms of revenue, the hardcore players are the core part of it; they also are the least patient ones. You will have to strike a balance between campaigns vs dungeons as the former really is a reflection of the latter. Most people complete campaigns for the rewards which dungeons are based on. If you wanted to test this theory, try introducing a campaign for people with no rewards i.e. boons and etc and let them enjoy the lore only. I would love to see how many neverwinter players actually complete the campaign. Dungeons with story embedded into them are very common in MMO's, maybe give that concept a try?
Some great commentary here and lots for me us to think about. Regarding narrative (although slightly off topic) we are going to be focusing on this more moving forward throughout the game. Specifically player relevance/purpose, a living evolving world, and interconnecting arcs of various levels.