OP, interesting variation on an old suggestion. However, I don't think it will work in the long run and here's why, long term profitability.
let's say Blizzard does reintroduce a Vanilla and BC server as they were pre-AQ40 and pre-SWP. I would imagine that both would be hugely popular from the start but after the nostalgia and curiosity wears off, both would soon begin to decline in popularity. Within a few months to a year, it will cost more to operate and maintain these realms than Blizzard profits from them.
I don't think the current generation of WoW players are going to go for spending 4 or more hours in some of the regular dungeons (BRD comes to mind) or having to grind out dungeon and raid attunements. Karazhan, there was a reason why PuGs never did the main instance before the SWP patch. It was also extremely rare to see a PuG player with a Champion of The Naaru title, forget Hand of A'dal. The average and current breed of anti-social WoW'er is far different from the classic social types that made up our 25 and 40 man progression and world PvP guild's.
Players are going to quickly get frustrated with the time it takes to farm gold and raid consumables. Anyone from Vanilla and early-BC knows exactly what I am talking about.
You would also very likely need to download a separate client for each, considering the game has changed so much over the years. Considering how much additional drive space that this would take up, some people are not going to go for this.
Some things are just better off left in the realm of nostalgia.
"a free man is he that in those things which by his strength and wit he is able to do is not hindered to do what he hath the will to do"
OP's idea is absolutely ridiculous. Playerbase fragmentation is such a great concern that Blizz introduces special tech to fix it (connected realms etc), and here you go, suggesting a model that would fragment the playerbases more than ever, completely artificially and without producing any more content? That's just... wow. I'm at a loss for words here.
Srsly, nostalgia or not, people need to stop dwelling on the past so much. Replaying same content again and again and again 8 years after it was current is your idea of fun, really?
I just hope Blizz will polish and release the 'Timewalker' feature soon enough, so we can actually downlevel and play some old content. This is as good as it could realistically get, without total and complete failure.