Actually, as has been found out in other games - if you make the game harder people don't 'get better', they leave the game and go play something else. Why? Simple - because unlike what you seem to believe, not everyone finds challenging/hard content fun or enjoyable, not everyone wants to have to stress and think when playing games... they just want to unwind from hard day/week of work and just play games for fun.
I always find myself sadly amused by people like you, who seem to think your enjoyment is the "correct" way and that other's just haven't found it. Here's the truth - people have found their fun and do what they enjoy doing, and no matter how much you say "but but challenges and hard stuff is fun!", they will not find challenges and hard content fun. It's not that 'they don't realize it's fun!", it's that they do not find hard/challenging content fun.
And sorry, but games ARE meant for fun and enjoyment. Just what people find fun and enjoying differ's from person to person. Some people find it fun playing hard/challenging things, some find it fun just doing easy stuff that doesn't involve anything hard/challenging. neither is right or wrong, just different.
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The problem for game dev's is how do you balance this? How do you have a balance of content that can keep the most players happy? Make everything too easy and those who like hard/challenging content will complain. Whereas if you make it too hard the players who like easy content will leave the game for other games that offer easy gameplay.
Moreover, if they try and offer a more balanced level where some/majority of content is easy while keeping some content hard(er)... they will still get complaints from those who like hard/challenging content as well as from those who want/like easy content. And this is where FFXIV is - the vast majority of the content is easy, with some harder content (EX's, Savage, etc)... and has complaints from both sides.