Look at Mario Kart Wii versus previous entries
I found the entire Mario Kart series to have roughly the same difficulty.
"Most" older games weren't difficult because of bad controls or poor collision detection; it was mainly just the bad ones that were like that. Plenty of games were difficult without having those problems. They still don't matter because we're talking about newer games, not older ones.
Super Mario Kart was a good game, but the steering was absolutely terrible on that. The original NES Megaman series (at least the first few) wasn't really hard, but unfair instead. Remember the Wall Shooter boss in Megaman 2? You needed to use EXACTLY all of your Crash Bombs to defeat them, and that's if you don't destroy the barriers first and die after. Nothing else hurts them, and if you run out of Crash Bombs you're out of luck because there are barely any weapon refills at the checkpoint you go back to after you die.
That is an example of unfair, not difficult gameplay.
As for your argument about regenerating health in FPS games, that's made so that players don't always have to go looking for health in multiplayer. Health packs would be useless anyway since you die so quickly. I do think, however, that regenerating health should not be in the single player modes.
Except the difficulty of games has changed. All kinds of things are done nowadays that make games easier, or worse, actively remove gameplay from the realm of player skill. "RPG elements," AKA grindable level systems and stat-based combat, are the biggest offenders and are prevalent in many genres today.
Yes, mechanic spoilers can make games easier, sometimes pathetically so. We've had that discussion before. But the Internet didn't magically make games easier.
What I think most developers are doing is adding more gameplay features. This leads the games to become easier over time since you have so many more options, unlike older games. If older games weren't as limited as newer games, they'd have the same exact difficulty level. You also have to take a look at a game like Super Mario World. You're equipped with two of the best power-ups in Mario history: Yoshi and the Super Cape. I haven't seen a recent game that allows you to skip through entire levels (excluding warps) as effortlessly as you can in Super Mario World.
There are, however, newer games designed to be difficult. The entire Ninja Gaiden series is difficult, from NES to present; Demon Souls is another tough game to beat.
My stance on this issue remains that, although newer games may bring in features that make them easier, they still have the same difficulty levels as older games in most cases. I can beat New Super Mario Bros. Wii just as easily as Super Mario Bros. 3 or Super Mario World.