the structure of the server changed so significantly that for current updates to actually work on an old framework, you would have to rewrite everything. You cannot insert an old piece of code or use it with the current framework, and by reverting the framework a lot of new pieces of code would not work. The references and the structure changed so much that it would bug out severely. That's why it's such an immense undertaking.
Yes, ofcourse you can 'simply' revert to the state of the server of 3-5 years ago, but this would remove a lot of content, fixed bugs and optimization that people are accustomed to nowadays. Not to mention all the work that would go to waste, all for the risk of gaining a couple extra players that will probably not come back because they have a job now, or because they moved on to other games.
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S Clegane