Mounts and Titles should only be available while content is current.
They took a step towards that with the addition of Ahead of the Curve / Cutting Edge FoS, and now with exclusive rewards like Kor'kron that won't be available after 6.0. Tie titles to Ahead of the Curve / Cutting Edge FoS as well and it would be fine for me.
Thinking now, there could be somewhere in game where you could see the name of all guilds who completed Ahead of the Curve / Cutting Edge, alongside all raiders names. As an example, there would be this board near that Tree that appears after Garrosh's defeat and anyone would be able to see all guilds who had cleared that. As this would be tied to Ahead of the Curve / Cutting Edge, clearing it afterwards wouldn't add your name there and your guild / raiders would have their name printed there "forever". Maybe just for Cutting Edge? Not sure.