There is a reason, it is not a good reason marketing-wise, this is the difference. It doesn't matter how many times you write that bolded part, it still doesn't make it true. Prestige rewards and achievements should be time-locked in order to keep them relevant. They aren't (and in the long term, it's probably better for the game), that's why they are meaningless when the tier is over. Now you can completely enjoy collecting prestige-less stuff if you wish, but that's not a reason to take out what others people got when it was current just because you didn't play back then or you weren't good enough. A very large majority of rewards aren't timelocked anyway and those who are are very easy to get anyway. And if you're not able to that means maybe you don't deserve them (which is a very difficult concept to grasp for the average WoW player, I agree).
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Will never happen. You can't recreate what was new 15 years ago, you can't go back 15 years ago only by scaling and reintroducing all spells / trees / balance.