AV was awesome, but probably a one-time chance for a different leveling thing. Also, people just leveled there because it was really fast.
Dungeons is very repetitive and with long queue (depending on your role), it is a very lame way to level.
Dailies basically time-gate leveling, and we all know what people think of time-gating stuff…
Class-specific questlines will not get you anywhere close to a good leveling experience. I’ve never done any of them while leveling, and nobody should.
Island expeditions are only available at level 110, and like dungeons are repetitive and may have long queue times.
So yeah… If we had AV available alll the time you could do what you mention. But other than that, I think nobody wants to sit in Orgrimmar/Stormwind waiting for LFD until 110 to sit in Boralus/Zandalar waiting for random island expedition.
At low levels, queue times are pretty fast regardless of role (in my experience), but it’s absolutely true at higher levels.
That said, this is really only applicable if you completed all other content. So at a low level if you haven’t finished every zone in Azeroth those are still available. With Shadowlands equalizing the leveling across expansions you could level through a particular expansion - and if you hit 60, could send the character to Shadowlands and whenever you are leveling another character, pick up the story right where you left off.
For players who have finished all content in all expansions you might be somewhat limited - but while dailies are time-gated leveling, they are also a big help when you can do them on multiple characters (and have it count to the same reputation pool to help your overall experience).
To be clear - this isn’t a perfect solution. You’ve pointed out issues that are quite valid. But if the other option is forcing me to grind the same story over and over again on characters, and the same rep to unlock the same essences (or items, or whatever is needed) then I’d rather wait for LFD and have stuff linked across my account.