This is incorrect. They have specifically states that lvl50 dungeons will remain in High Level Roulette, while 51-59 dungeons will be added to the Low Level (or leveling) Roulette.
Ty, wasn't sure about it, i guess we will be able to solo for some tomes(knowing SE no more than 10-15/day), but must be a bit hard for leveling purposes, and we don't know for sure if high lvl roulette will be locked for premades.
So far speculation on relics has been that those who have completed their zodiac prior to HW will get some sort of consideration and that may be with regards to foregoing a step in the process of upgrading. Others have mentioned that there is a new relic 3.0(1) that will not need prior relic to be completed. However what is said here is the the new system will help us get the items needed to upgrade Zodiac weapons.
Why would anyone do that if there is just a new relic at 3.1? Unless it is required to upgrade our relics to Zodiac in order to progress it in HW?
The part I underlined is pure player speculation. "Taken into consideration" can mean anything, even simply meaning that an NPC involved in the new quest may use a different dialogue which acknowledges the fact that someone completed their relic quest. At that point the accomplishment would be "taken into consideration".
Anyway, to answer your question.. some people may want to complete the (soon to be old) relic quests because they want a particular glamour, or want to experience that part of the storyline, or are simply completionists. Keep in mind that "zodiac weapons" include Atma, Animus, Novus, Nexus, Zodiac Braces, and Zodiac Zetas.