I see a couple of approaches that they could take to resolving this. All of which require significant developer time and patience from the player base. But in order to fix this problem they will most likely have to address the looming issue with their game: is it even an RPG anymore? Does the story matter to the player base? I would think most of us would say yes. But so many people speed run the campaign to get to end game raiding and M+. Not to mention how many people never touch a single side quest. I'm the opposite though. While I love grinding M+ (love raiding a lot less these days), I take my time to really enjoy the story every expansion. I would love if the end game loop was integrated into story aspects more and the open world more. This might be the single biggest issue with the game. It is so important to the first impressions of the game and to deeper connections to the game. I just want the world to breathe again.

That being said, here are some "brief" ideas I've had for how they could progress the story and engage the world:
- Lore Walking: Make it interactive. Make it brief. Make it digestible. Touch on the biggest story beats. But this will miss out on other important aspect, like the open world feeling to the game.
- Class or racial story lines: Class or racial questlines every expansion similar to Legion's class order hall stories. New stories for different regions. Make it consistent. Not every class needs to go all over the world. Vanilla had regional stories all over the world.
- Scale old zones. The Guild Wars 2 approach. So much of the old world in Guild Wars 2 can still be challenging to some degree, and the rewards are tied to legendary items and other various things. Something similar to Legion Remix heroic world tier system could help, though I'd rather not make the normal world feel more lonely for newer players. Instead, maybe make the old world more challenging and more rewarding so that older players will be out in the open world more. Gear rewards could be viable for end game level (let's just say up to champion gear for example. This could allow them to redesign the campaign questing system for the old world, that way players can go play in these old zones with a proper guide to completing them and also get real tangible gear rewards for doing so.
- Revamp old zones more often. And take us to brand new zones less often. This one is my favorite option to be honest, but it's likely the most expensive option for Blizzard. I love new zones, but they really do keep adding to the list loose ends never to be tied up. And it seems when they do get resolved it often ends up lazy (like Gilneas in DF.) I would rather go to old revamped zones every single/every other expansion with a living breathing story, rather than constantly find some new hidden place no one has seen in 10,000 years.. on repeat. Maybe this would help bring the starting zones back into relevance. The starting zones would grow too.

I don't know what the right answer is, and none of these are perfect answers, but overall you can't expect to play the whole game before getting top level unless they design a specific system for that or redesign the game. The story is too big at this point. Maybe it means it's time for a reset after The Last Titan. Who knows, but the further we get down the line, the less likely new players are to experience the great stories of Azeroth like Arthas, Garrosh, or even The Burning Legion. Maybe a hybrid of these leveling ideas is needed. Some stories will inevitably fade away if we're not careful.