Not surprised. It'd be stupid to drive casuals away from big Nintendo IPs by making them too difficult. That said, I'm not exactly sure what's changing here, as Nintendo first party games have never been particularly hard.
This actually seems to me to be a trend we've seen in the industry as a whole. After the PS1 made casuals so important, difficulty in games took a nosedive. The best that a gamer looking for a real challenge can hope for is an option to play with half as many HPs - a 'Legendary' mode, essentially. With the sole exception of Ninja Gaiden, I can't recall a recent game that was built to be difficult.
It doesn't really strike me as novel alienation of core gamers, looked at that way. To some extent, it is, but it's no more alienating than what every system did last generation, or the one before that. Core gamers have long been perfectly happy with all sorts of games that were, at heart, absurdly easy. RPGs have always been lacking in challenge, FPS games and assorted hack-n-slashes have always relied on artificial-feeling 'difficulty modes', platformers haven't ever been difficult for a regular gamer, etc. Game difficulty has catered to casuals since we left the arcades.
Part of the reason is probably that the 'sweet spot' of challenge is virtually impossible to hit for different people. If you make the game too hard for someone, you've lost a fan - he'll get really frustrated and angry, and will blame the game for poor design. You don't run nearly as much of a risk by making the game too easy, and this is the route that the vast majority of developers take.
Often, when we say that we want a difficult game, we're saying that we want a game that is almost too difficult for us. This maximizes your sense of accomplishment by maximizing the number of people who can't replicate your feat. However, Ninja Gaiden, for example, takes a lot of flak for providing just that to a certain kind of gamer. Many people think that it's just ridiculously hard and frustrating.
There's always going to be a market for very difficult games, because certain core gamers are going to make it a point to buy and beat them just so that they can call themselves hardcore, and those who are good enough at such games are going to legitimately enjoy them, but for over ten years now it's been best to design games for the least skilled player.
To sum things up in one sentence: High difficulty is only an important part of game design when your goal is to get players to burn quarters buying extra lives; it's actually detrimental to a game's success when success is measured by the number of people willing to pay $50-$60 for as many lives as they like.
