I think most of the people (myself included) that think the games are getting easier are those that can play through the hardest difficulty settings without too much difficulty. I don't think anyone is arguing that there shouldn't be an easier mode for people to play through otherwise goldeneye levels wouldn't be referenced so often. Agent on goldeneye could make you feel invincible after playing 00 agent.
Levels used to require multiple tries and prior planning because sometimes you had to think how to deal with what's ahead of you. I've played through all the halo games on legendary and never once felt I couldn't get by just winging it (granted I haven't finished the halo 3 campaign yet as I've only spent about 2 hours on it so far). Playing through COD4 on the hardest difficulty I beat it in less than a day and only died on a few parts. The only level that really felt like a challenge was the bonus plane level at the end. Goldeneye made me thing about which enemies to take down in what order, what cover to hide behind and whether I'm better off charging forward or advancing slowly. Halo I can just charge forward and if it ever gets too tough I can just duck behind whatever is close.
Maybe it's also because I'm not that much of a halo fan, but the skulls for me never felt like they were part of the game, but instead just cheats that make things more difficult. Skulls are something that requires you hunt down and find them before you can use them. It just doesn't occur to me that I should play through on, well I guess you'd consider it an easier difficulty, just to hunt down objects in obscure locations so I could play back through the same thing with a challenge. That and even since goldeneye games had things to make the game harder that you could turn on, but weren't considered part of the difficulty of the game. Saying play legendary with all skulls on just sounds like playing goldeneye with enemy rockets turned on. Sure, it gives more of a challenge (00-agent train level is impossible (well unless you are this guy) but people don't usually consider this part of the difficulty of the game
Easy difficulties are good so that everyone can play through the game, but on the hardest difficulty there is nothing wrong with making it so hard that people get frustrated and have to get up and leave before they try again. Sure, some of the audience might get frustrated and give up, but some of us liked trying to overcome levels that seemed impossible like keeping Natalya alive or the water level in teenage mutant ninja turtles or pretty much all of ninja gaiden, because as angry as it makes everyone to actually fail to beat it, it feels that much greater when you do. There is nothing wrong with having levels that 90% of gamers can't beat on the hardest difficulty. That's why its the hardest difficulty.