You don't have a clue what you're talking about. Even if the code still exists from back then, they still need to re-fix 10 years worth of exploits/severe bugs, and it will be very difficult to do so, since nobody will be familiar with the old codebase. Their infrastructure has changed completely from back then, it wouldn't even be compatible anymore. It would require a shit ton of testing, even more than a new expansion would. We're talking about Blizzard here, they don't release half-assed products.
And it's much more effort than making a new expansion, since they've streamlined their development process so much, there's really not as much that needs to be done. They've built tools to make creating content as easy as possible.
It is not minimal effort.