No it wouldn't. Tier 3 did have value back in the day, though. It increased your stats and helped you perform your role better as you progressed through Naxx. After that, it was just another set of armor that looked good to some and ugly as sin to others. My set of Frostfire pieces didn't decrease in value when they started appearing on the BMAH. So why should I care if someone that started playing the game in MoP likes the look of the set and buys it off the AH for transmog? Good for them if that's what makes them happy.
As for mounts, their only "value" is in whether or not you think they look cool. So you were a Brutal Gladiator once? Cool, no one gives a fuck. So you think your Brutal Armored Netherdrake is impressive? Nah, the only good looking one was the Merciless one. Should you be impressed with my Furious Frostwyrm? Of course not. I personally think the Wrath PVP mounts are inferior to the armored ones, but if someone else wanted to fly on the PVP versions without having done PVP once in their life that's fine by me. Doing well in S6 meant something in S6. It doesn't mean shit now.
I hate the idea of retiring items, mounts, and titles. The achievements are there to remind yourself of what you've done. The rest should remain available one way or another for anyone that wants them.