I feel like the issue is less 'the game's content is not hard enough to force people to learn to play their class effectively and figure out mechanics'. I think the issue is more 'the game's content is really bad at instructing players in their class/job and role'.

The Hall of the Novice is great for teaching the absolute basic concepts of a role. If you rely on it and your class/job quests to learn how to tank, however, you're probably going to be a mediocre tank at best. Not through any direct fault of yours as a player, but because the game has not really given you the tools to learn to do better. That tank you had who isn't holding aggro well? What content in the game taught them how best to hold aggro on multiple mobs? What content actually demonstrated to them that single-target rotations without AoE are going to potentially backfire if DPS rips the mob away from the tank? What content even taught the them the appropriate use of cooldowns?

For that matter, what content even taught them how to read the aggro table? I've taught that one to at least three sprout tanks in the past month, all of whom were like "this is a revelation and makes my life so much easier".

To use an analogy, imagine you want to bake a cake. It's your very first cake! You have an Easy-Bake Oven with little packets of cake mix, and you put that into a pan and it goes through the little oven, and you have a cake! It's not a great cake, but it's a cake you made. Now you want to make a better cake. A real cake.

You'd go use a real oven to make a real cake, right? Maybe later an industrial kitchen oven if you wanted to get into making those spectacular cakes they show on the Food Network. The problem is, the game doesn't give you those. In fact, the game doesn't even tell you other ovens exist. Some folks go out and find better ovens anyway; they hit up the Balance, they ask experienced players in their FC for advice (finding guides for 'how to tank in Heavensward content as of patch 5.1' isn't really easy), and so on.

But for the folks who don't have that network? Who don't even know the Balance exists to go look at it? They don't even know that other ovens exist. They just know they're trying to make a nice normal chocolate cake in this Easy-Bake Oven, which can be really hard, and sometimes other people go "Your cake really sucks. How does it suck this bad? It's a simple recipe."

Turning up the difficulty of MSQ content and disabling the 'Very Easy' mode on solo duties will not appreciably improve the skill of players, merely frustrate them further. (And it will block folks with legitimate handicaps from progressing as easily to enjoy the story, which is an entirely different topic to discuss, but one worth noting.)

If you want to change things, I think you're starting in the wrong place. Address the game's woeful lack of any really good instructional mechanism for players who want to learn and improve, then consider tackling increasing the complexity.