Yeah, this idea of Classic+ is not really workable for that reason, but also because it means that this small team becomes a big team, because now it needs designers, engineers, artists, animators, etc., plus lots of additional QA resources.
So if you’re running WoW, you can either put those resources on Classic or on retail WoW. I suppose if after 4-5 months Classic is somehow way more popular than retail you could possibly make that argument, but with at least two expansions worth of additional content available nearly free for Classic, why would you?
Not to mention the outcry from the player base that now is forced to play Classic if they want new content. Believe it or not, but lots of people are just not into it.
The most obvious path is to move on to Burning Crusade after Phase 6, not add new content. New content is for Retail.