I think most people put weapons in a different class than gear. Relics have never been outdated till Abyssea, before Abyssea Relics were high end and took a long time, they were always at the top of the latter besides Mythics, and some specific weapons for jobs with bad relics, like BLM. What I mean to say by all of this is, while normal gear like bodies and such are outdated most of the time, Relics never were till Abyssea, Mythics never were till Abyssea, and guess what? They updated them too so we could keep them & continue to use them as we progressed. They have never outdated RM weapons and left them for dead, why should they do anything different now? RME weapons should be updated for the fact they have never been outdated before, no expansion has done it till now and gotten away with it, I highly doubt this would could get away with it, and since they already announced they will be updating them, it seems we get them updated yet again as per request.
Long story short, they do not out date RME level weapons every expansion.