Early on? Are you kidding? I'm not talking day 1, I'm talking: while it's still current content - before release of the next raid tier.
Horde is at 120 guilds palace Mythic while Alliance is at 23.
This "requirement" is not only punishing Alliance (the dev's favorite occupation appartently) but also Horde players on low pop servers. Since the basic mythic strats are out within 2-3 weeks of release, what's the point of blocking cross realm with arbitrary requirements.
Just timegate this shit. 2 or 3 months after initial release is not too soon. It's not like there is any mystery or secret any longer since the mythic progess is streamed live nowadays anyways.