That is -alot- of content to snip, though. There are also lots of callbacks to older content that would also get missed if you didn't have context. Even tiny stuff, like the Starship Trooper parody gnomes from Burning Crusade showing up in BFA. On the flipside, though, with accelerated leveling you aren't going to see lots of the old content unless you are a completionist. I skipped WoD and when I had to level through it on my alt (used level boost for my main), I pretty much only saw the first zone and got the fuck out once I was high enough to use Legion content for leveling.
However, the thing to realize is that old zone revamp is unto itself a full time job. This happened in Cata when they revamped all the Vanilla zones to bring them to a more modern state in terms of questing and graphics. They have gone on record in the past to state that revamping BC/WOLK/etc would be massive undertakings and considering the rate of leveling through that content, would it be worth it? The reasoning is why should they spend millions or massive manhours revamping content that people are going to blast through in a day /w accelerated leveling?
The only way I see them doing a MASSIVE rehaul is if they see a large decline in subs and determine the only way to save the game is to "start from scratch". They'll probably go the Square-Enix FFXIV route and add a paid "story skip" option that acts as a way to bypass older content that takes too much time to progress through.