There shouldn't be a Myth cap because.....

[Amyas]

SE is backwarrd people who only play one job are already all lv90 but it take weeks to get lv90.
i play 3 jobs pld, mnk, whm so im going to be left behind.

they just need to add actual content to gain said items like relic quest had something more then a simple drop, not a stupid lazy time buffer to extend how long it takes. time sink =/= content or fun

I play all jobs, with 6 in i70+ and relics, i90 on my main which is war. You don't really get left behind if you play your cards right.
edit: I'd gear your pld first in myth and relic, the rest in DL/primal weps until you feel you can sacrifice some myth for em or farm other high end content for i80/90 gear.