Also, even if it opened straight after the last boss died, that'd be fucking terrible for most guilds. Imagine if a boss lives 1-2 resets, but you have a roster with 5-10 people on different realms. Either you need them all to transfer for the start of each tier, or you would have to start mythics 1-2 resets late. Not to mention it removes even more of the "MMO" feel of the game by not allowing cross-realm guilds.