As you know, making demos are sometimes frowned upon in our industry, with the claim that they lower sales.

In depth look 

But in the case of games that are 60fps, demos can make games shine better than any trailer because youtube can only do 30fps. So people actually playing the demo coming off watching a trailer can go "Wow this game plays better than I thought it would" and be impressed and more inclined to buy the game.

It would really help some of the more niche games in particular. (Looking at you Wonderful 101, I'm not sold on the game until I get to play it myself)

No, because if the game sucks, no one will buy it.

60fps is nice to have and I do appreciate Nintendo's dedication to having most of their games run at 60fps but not buying a game because it doesn't run at 60fps or being pushed to buy it because it does seems kinda strange to me.

Your game could have 9001 FPS I don't even care. If it sucks I won't buy lol.