why do i have to be on level 60 class to get level 60 currency rewards

[Nezerius]

Just like how Scripture will remain so that those who only have ARR + HW can purchase endgame tomestone gear.

Actually, if the previous live letter is any indication (specifically the part quoted by Rongway on page 1), then Scripture might not remain with the release of Stormblood, and the gear bought with it will be moved to the Poetics vendor.

Unless "Furthermore, starting in 4.0, all previous tomestone exchangeable gear will be available via poetics." is talking about all the gear previous to scripture tomestones.