I used to be involved with this sort of thing, and it's actually pretty interesting stuff. There are enormous open source projects, operating largely out of public sight, that have recreated the WoW server backend in most of its glory. The lower-level content works quite well these days, though a lot of complex raid scripts are missing or broken. This one is probably the most popular: https://github.com/TrinityCore/T…
One could argue that the Lich King was the last "good" patch for WoW. What I've never understood though was there is clearly some residual demand for "down rev" versions of the game where the mechanics favored a more engaged play style, why not facilitate it? How much does it really cost to run a single realm at the Vanilla, TBC, and LK levels? So many new players where their first experience with Onxyia was soloing…
However, it's worth mentioning that Runescape now officially offers a "classic" version of the game alongside (and separate from) the modern version, which should be an interesting experiment to watch.