On the whole, I've always found the "main" games pretty easy. I recall struggling some when I first played through Red, mainly because of my difficulty getting through caves/dungeons. Indeed, I remember restarting my game because I wasn't sure how to get through Mt. Moon. I also recall attempting to Struggle myself to death to get out of the Seafoam Islands (I eventually stumbled across an exit, but with an overleveled Blastoise that only knew offensive moves, all of which OHKO'd any enemy I encountered, I couldn't come up with a faster way to knock myself out). In retrospect, though, the games were only difficult at first because I was young and didn't really know what I was doing.
At some point, several years after I first played Red, I went back and replayed the game. It was interesting to note that it took me about ten to fifteen minutes (while half-asleep) to clear a dungeon (Victory Road) that I had become stuck on for months when I first played through.
The newer games seem easier to me than I recall the older ones feeling when I first played them, but that's likely a product of my increased experience and abilities. Even if the games have shifted from being moderately challenging for, say, an eight year old to being equivalently challenging for a ten year old, I've grown a lot more than that in my fourteen years of playing Pokémon. Frankly, given that Pokémon games are likely geared towards a relatively young audience, I think it's a good thing that I don't find them challenging anymore (particularly in comparison to games I've played that are geared towards an older demographic). Many posters on this forum are likely in a similar position, and hence, the majority of us probably find the games easy. If this forum were made up of eight year olds who had never played more challenging games, we would likely see a different distribution of opinions on this topic.