From developer's side there's no point to keep old, irrelevant content dead by gating it behind original difficulty. Nobody would do it now so these raids would be just dead files on everyone's hard drives.

I get that the early clearers want to keep their achievement rare as a proof of their skill, but no MMO works like that. It gives them their 15 minutes of fame then allows everyone and their mom to repeat it a year later.

If you want the challenge, do all content when it's relevant. Because soon everyone will catch up and cheese it by massively overgearing.