Classes are not sub-max level content. Aside from the DK starting point, which only existed due to the starting level being at 55, classes themselves require nothing other than simply being. There is nothing to say there won't be new races and sub-expansion zones. For those that want to see them, the zones will be there. And you'll still be able to boost your character to 90 after you're done. The dungeon revamps only exist because they've been made into Heroics, for max level. So that doesn't apply at all. And who cares about Heirloom gear? MoP didn't have any Heirloom gear aside from the weapon that drops from Garrosh, which is only beneficial in the NEXT expansion. I mean, the entire reason Heirloom gear even exists is to help alleviate the issue that they're looking to eliminate with selling character boosts. You know, leveling.
Now, as for Pet Battles. I listed it, but it doesn't apply. Since it's not specifically sub-max level content. Pet Battles aren't level dependent (unless your level is too low to advance to the next level for pets). Boosting a toon to 90 doesn't suddenly make them unlikely to put pet battles in older zones. As for the zone revamps for Cataclysm, it was a colossal waste of resources that didn't achieve what they were hoping it would. Yes, it streamlined the leveling process and made it more accessible. No, it didn't do much to deter people who saw a game on its 3rd expansion and said "It's gonna take me forever to level." As for Dustwallow Marsh, it fit the story, so they did it. People leveling through wasn't the reasoning behind it. It wasn't content specifically created for anyone under max level.
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The boost that comes with the expansion is far more likely to create a population boom. It will appeal to both sets of players, old and new. It'll appeal to the old players that don't want to have to level 20-30 levels to get to max level to enjoy the content. It'll also appeal to the new players that look at "100" along with a subscription and say "No thanks. I'll always feel behind." I've said this numerous times, but WoD is almost like a reboot. Yes the game and the engine are the same, but it's allowing every single player that picks it up to be on the same playing field.