Why not unlock myth and BC now?

[Gamemako]

And in another separate Japanese interview, it is also confirmed that myth tomes weekly cap will also be unlocked. Basically all i90 related content will relegate to Darklight status because they will raise the new item level cap with new set of BC loots, Leviathan weapon and new tomestones.

Not likely, as much as you like to read into it. Every single pre-90 dungeon would be fully obsoleted for all purposes except LotA. They would completely destroy all uses of WP, PS, CBHM, and HMHM -- all of them would never be run at all for any reason. AK and HM primals would be run-once for relic quests. Gearing in i90 would take a weekend, and all content in between would be rot.