As someone who gets easily frustrated I almost always play on low or regular settings on any game I play (at least the first time through). While I used to enjoy spending hours repeating runs on sonic 2 trying to finally reach the end, I can't honestly say I get the same feeling playing through so many years later.
As I grew up and broadened my gaming horizons I soon felt cheated when a game was difficult to the point that I had to replay levels multiple times. Sure Every play I would get a little better, but I'd also lose a bit of my sanity after failing for the umpteenth time at the same exact spot. Nowadays I just play games to enjoy the ride, not to challenge my hand-eye coordination skills.
Now the only way I will play sonic 2 is on my 360, despite access to a genesis copy of the game. Not because I'm lazy, but because with the XBLA version I can save at key points and load if I fail instead of starting on green hill zone one again and again.
I like being able to play through the hot new blockbusters without having the stress of dying and reloading. I would consider myself a hardcore gamer not because I'm anything special when it comes to playing, but because my love of gaming keeps me playing for hours a day. I can understand how some people might miss the difficulty back in the day. For some people games are all about the challenge and there are still challenges around if you look for them (Guitar Hero, Ninja Gaiden, F.E.A.R)
There's also the problem that difficulty can be extremely speculative. What I find difficult others might find a cakewalk. Everyone from the most casual to the most hardcore of gamers have different experiences with difficulty in different games.
Then theres different difficulty levels. This is a developers way to appeal to both parties of this particular debate. Unfortunately sometimes this doesn't work out very well for those wanting a legitimate challenge. A lot of the time the only difference between easy and hard will be the amount of enemies thrown at the player. Usually it adjusts player and enemy health. The problem with this is that a lot of the time this just creates an artificial difficulty. Take for example CoD:WaW. I found the game to be perfectly balanced on normal. I only died occasionally but had a lot of close calls. The problem lies with veteran difficulty. Instead of making the enemies smarter or tougher, the developers created artificial difficulty in the form of a grenade spam. I think most people can agree that this wasn't a challenge, it was a frustration. The game wasn't designed to give you a legitimate challenge on veteran, so the higher difficulty suffered.
Ultimately it comes down to developers. In order to survive they have to appeal to every player they can. If in the end this alienates the challenge loving gamers out there it will only be there fault. I truly believe that a balance can be found where Easy=Everyone, Normal=Experienced gamers, and Hard=Challenge Lovers only.