I think with MN HS hockey, the point is that even if it's not of a particularly high quality, it's still far better than the high school hockey you see in most states in the country. At my high school, if you wanted to play you had to play for a different high school entirely (that team was actually comprised of kids from four high schools). Even public schools in MN are substantially better they are than in 90% of the rest of the country. Of course they are not on the level of the prep programs, but public MN and NE HS programs are still better than what most people know from their own upbringing.
One of the reasons for the "decline" in non-prep HS hockey is the rise of the USHL and NCAA. For a long time, if you were an American kid, you could try to play Canadian junior or you would play HS and then go to college. The USHL and by extension the USNTDP has grown by leaps and bounds in the past twenty years and is a much more attractive option for kids than it used to be, to the point that the USHL is even considered a viable option for Canadian and European kids. Same goes for NCAA hockey in general.