Indeed they NEED to think of a solution in the long term since story content will keep getting bigger and bigger and catching up to whatever the current expansion is, will take weeks if not more (for casual players).
I don't know of a solution, but "skipping" it makes no sense in the current context, the story of 3.0 onward makes little sense without knowing what happened in 2.0.

Maybe once an expansion is out, leveling a new character could have the option to do only certain quests, basically the relevant highlights of the older expansions and the rest of the MSQ fillers) could be presented as voiced slides in-between the key points. It may sound like a lot of work but it isn't really, not compared to the benefits of opening up the game to new players in later expansions.

The lazy (imo) and cheap approach would simply be to allow a player to skip every single quest (consider it made) up to the current expansion's first quest, unlock all cut-scenes and such, also unlock absolutely all dungeons automatically as you level up (including the side-ones, you can do the quests but they're not mandatory anymoe) from all older-than-current expansions.

There are ways, ways that would impact the older players very little.
New players that care for the story should be allowed to experience it however, if the choose to. Removing older content for "speeding up" reasons would be terrible.