I've been doing a couple of the ShB ones while leveling up some of my Jobs. They do still get run by players doing the MSQ at least. Just scaling them up wouldn't really work, Blizzard scale their dungeons to Heroic then Mythic difficulties. They're not intended to be run at 80 beyond initial gearing but are used as building blocks for more advanced content.
Without that advanced content, you're running headfirst into the same situation where you'd run the same small pool of dungeons over and over.
I think the best solution would be to increase the number of Tomestones you get from activities that are not the Expert Roulette. It won't magically add more content, but it will at least give you more diverse content to choose from.
The Dungeons that really tend to get forgotten about are the ones that were max level dungeons, but not a part of the MSQ. New players don't often unlock them, so even queuing on the 50/60/70 roulette you're not likely to see them come up often.
As for how to fix it... I'm not sure really. Maybe have them unlock automatically when you get to the appropriate level? It would diversify the pool at least. They could do the same thing with Trials too, since a lot of them are locked away behind quests players have no real incentive to do.