I've been leveling my alts this way for a couple years now. I agree that it's much more fun than using LFD, and I try and talk friends/new players into tagging along with me whenever possible because I do feel it's a much better leveling experience than facerolling junk. There are a couple weird scaling issues at certain levels and whatnot, but overall, it's a breath of fresh air-- especially if you go with a non-standard duo like a hunter pet tank or whatnot. I also find I learn a ton about the class I'm playing while doing this, which is great prep for endgame.
I agree that it's a shame that more folks don't choose to do this/that this isn't the default way to experience dungeons. Some players are turned off by the fact that you have to actually go to the dungeons themselves, I know. Others, of course, just want to whip through leveling ASAP. I sometimes feel like I'm really old school when I go on about how much fun I have doing stuff like this, haha.
I also agree that we'll be seeing scaling tech for all dungeons/raids soon. Timewalking seems to be popular enough for sure. It'll be interesting to see if Blizzard just keeps with the 'limited time" trend for scaling tech or if they go with something like FFXIV where you can always get benefits from running older dungeons w/the scaling feature enabled.