Well sometimes that is due to using mechanics that no one but that public likes =p
True, and as a fan of turn based strategy games I definitely know that from personal experience too
You don't need to appeal to everybody, no game will be able to do that. But if there are people that like almost everything about your game but don't enjoy it because of the difficulty, then you screwed up. It's as simple as that. Adding difficulty levels shouldn't be hard, and it certainly isn't going to ruin the game for other people.
I mean, look at FIFA. That's a game that some people are ridiculously good at and play professionally, does the fact that the beginner difficulty is easy enough for even a 3 year old to win at ruin that game for those people? Absolutely not, they just play on legendary difficulty (or more likely online multiplayer) instead. Or as I said I'm a fan of strategy games and Civilization has like 10 difficulty levels, that just makes the game better. Not only does it mean more people can enjoy it, but it also means that as you get better you can up the difficulty and still be challenged.
That is a good example, and I hope FFVIIR make it a good case where both turn based and action are plenty good so more games can use more than one mechanic.
Another problem besides people liking almost everything on the game except dying all the time and having to replay all, or grind for the fuck sake to correct the step in difficult, is that it could be like 10x more people than what currently like the game as is but the ones that like it don't accept some improvements for those people to enjoy the game because they would lose the status he attained by mastering that game.
I love the fact that I platined GoW on Give me God of War, and don't feel any less valued because several people have done it on the Give me a Story (well some couldn't even finish the game on this difficult, and that was really easy =p).
Like Resident Evil, I don't like the fact that you basically have to get S+ on the most difficult setting to enable the best cheat weapons, people that can do that don't need the cheat weapons. Just disable the trophies adequately (if at all) and allow people to decide if they want to use those weapons (they rather do knife only or no heal runs). Lock clothes and lore as achievements for finishing on the hardest difficulties with good grade.