For those of us playing for story over supremacy, and in a more casual position, it's the best idea I've heard in a long, long time. I'm actually not that interested in leveling alts just to have max levels; I've got a group of toons I'm trying to keep up with the wave, and I'm kinda maxed on the number of them I'm willing to do it with (for me). Catch-up techniques are keeping that going. And you can keep the dungeon-grinding option for the folks fleshing out a class for max raid play.
But what about new classes? I want to get connected to that character by playing a story with them. It's only been when I can connect a story to the new character I've been inspired to put in the time. And I've been having to make up that story myself, or look up how to make sure I wasn't missing the story beats as I went through. I've got a mage stuck in Wrath because I want to play it, but I ran out of gumption to keep trying to look up the story links in Wowhead. And I discovered I'd probably level out of Wrath before I could finish. So I'm stuck.
I didn't get all the classes through Legion's class stories; I want to see those. I'm feeling nostalgic for a lot of the story lines from Wrath and Pandaria (because it's been long enough), and I'd be willing to play 15 hours to go through them, if I'd get to see them all. I mean, at max level, you could run the raids anyway, but the x.2 and x.3 and x.4 story lines wouldn't be there. And I'd miss them. Well, some of them; the busy-time currency grinding between raids, no thanks.
It was only Legion's class story that got me trying to expand my roster of max level characters. Hell, just to complete the class story for every class, I'd probably do Legion a bunch more. Doing it now, I have to leave, go do something else, max level, and come back to the raids. And that still misses the zone stories. I'd much rather know for the time invested that I'd unlocked all the zones, gotten all the flight points, hit all the story beats and phased all the phasing while playing the story. This is for all us "Loremaster' player types.
My prediction is that Blizz will get to this path, probably not by Shadowlands release, but probably very soon after.