Galadourn wrote:I find it VERY hard to believe that a company the size of Blizzard has not kept the code of each and every iteration of the game archived somewhere.
They have. It's not about code, it's more about changes made to their server infrastructure, release rollouts, integration with Battle.net, etc. That was not a thing back then, vanilla and TBC basically was its' own infrastructure distinct from Battle.net.
Also back then code was heavily optimized for a specific type if rack server (HP Proliant), and now hardware and data centers have changed a lot. Performance on current hardware may fail, or even building may fail.
Just take compilers... C++11 was a not even in sight back then, now it presents a whole different landscape, many compilers even have obsoleted what was built in 2005.
In the end it is not all about having the code. If you take a piece of software from a century ago, it does always require time and effort to update it for the current landscape of development and systems. Legacy software often means a lot of effort is involved to make it run again, as what it ran on in the past is simply gone.