This is something that people really need to start to understand, not just for Classic or WoW but software developement in general. Fixing a bug isn't just flipping a switch or changing a single digit. You have a machine with literally hundreds of thousands of parts, designed by dozens of different people at different times over the course of 20 and more years. You change a single thing and it might break 50 other things in very subtle ways that might not even emerge until months later (i.e. the very high M+ keys some players got at the start of BfA).