I don't think they have gone away mate. I think some people's idea of what mid tempo and beat ballads differs. You still hear the likes of Tommy Ridgely, Eddie Whitehead, James Lately, Nu-Rons, John An The Weirdest and many more at all-nighters. Maybe not at Kings Hall but even there I dare say they play their fair share. Butch has some great mid tempo in his set, had me up to the decks a few times in recent times. Andy Dyson, Mick H, Kitch, Sam, Arthu Cliff and more still play mid tempo so maybe people should choose their venues a bit more carefully.
Even Burnley plays its fair share despite what the uptempo terrorist Terry preaches.
Beat ballads is a different case, they have disappeared, not entirely but you might get two a night whereas in the 80's it would be two per set, not as many kind as people think there was.
I might go through the Stafford stuff at the weekend and pick out 100 influential/great mid tempo numbers.