Back when Molton Core was the first raid in WoW, Blizzard ‘nerfed’ it by introducing fire resist gear. These days they nerf raids by turning god-mode on for everyone and letting them breeze through the content. Trion also elected to nerf content rather than provide content-specific paths to power.
Of course the first method also introduces new content, even if it is just a separate gear grind that’s only useful for one raid. The upside here is that players can decide how far to go down that path, and in turn how easy to make said raid. If you already had MC on farm, you did not need the new gear. If you struggled on Rag, you could always equip more people with fire resist gear to up your chances. Keep gearing people up until you down him, then move on.
The addition of a side-grind is less entertaining than a whole new raid, obviously, but given limited dev resources, I’d rather get a small patch with a side progression path than one that just outright makes everything easier by nerfing the mobs and buffing the players. From the “keep them subbed” standpoint, a side-grind accomplishes this goal better than the nerfs as well. Not that this fully explains why WoW was able to retain players so well back then, but it certainly helped.