This seems to be a symptom of a part of endgame that was meant to be in since launch having been postponed. If Myth gear is iLvl 90, and Coil drops iLvl 90, then it would be reasonable to assume that while iLvl 70 might be 'gimping' your party, iLvL80, while 'not as good', would be the low-end starting point for Coil.

If it was possible to run a dungeon that dropped iLvl 80 gear for a chance to acquire said gear as fast as or faster than it takes to grind 300 myth tomes a week, with the end result still being that you can enter CoB, then suddenly it makes sense that there's a weekly cap on myth tomes because running CoB for the same ILvl gear before you even have it would actually be possible, and the tome grind option would need to provide an equivalent wait since you can run dungeons like AK and WP more than once a week.

If I'm not mistaken Crystal Tower is supposed to drop ilvL 80 gear.