It's true that devs have their data but don't assume they know everything just because of that. They are still humans and not perfectly objective machines. They have their own view of how things should be and that may not always be what the majority agrees with.
If they did, things like the signifcant drop in healer mains with ShB wouldn't have happened. It's not just a vocal minority in the forum, it was and still is plain as day because healer queue times have never been shorter and they are often the last slot to fill in PF. No matter the hour.
And it has been critizied ever since they first got rid of somewhat challenging dungeons that this game lacks content that is the middle ground between casual content and endgame content.
A system similiar to M+ from WoW would offer precisely that. And it has pretty insane longevity for much smaller design costs than e.g. a savage tier.
Besides, while the first level would likely barely be more difficult than current dungeon iterations and thus easy enough to pass for random DF groups it's only at higher levels that there is a signifcant risk involved and mistakes get more severly punished the higher you get.
But you don't expect to queue via DF for a M+ 10 dungeon and clear it by rolling your cat over your keyboard. It's a smooth increase in difficulty not some insane jump from faceroll to savage like difficulty you seem to assume.
You could queue for lower levels via DF and likely clear it without much trouble as they are supposed to be your starting point and will switch to PF and mini statics for high levels.
It's not mutually exclusive with our current dungeons.
As Liam said, dungeons don't have to be casual only. Raids have a normal mode, dungeons can get a "savage" mode.
It can offer alternative way of gearing up, it offers challenge outside of ex and beyond and the best part is that you can always decide how far you want to take it in terms of difficulty.