I suggest leaving the Mists CMs

I suggest leaving the Mists CMs there for people to play through for years to come, but remove any reward for bronze and silver and make gold reward the mount but no transmog gear or title, which will disappear come WoD as a reward for those who earned gold during Mists. The guild gold medal achievement and corresponding pet should also remain to encourage more guild activity and social camaraderie.
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I think Blizz can easily reward current gold medalists with exclusive xmog gear and a title while still letting other players have fun with the Mists CMs in later expansions. Thanks to item level scaling they'll still be somewhat challenging and fun, so why not let players who are late to the party experience the dungeons and get some lesser rewards, like the achievements, a mount, and a guild achievement pet?