Unless there is a real big change in tanking mechanics, t6 will still be "bordeline ready" for heroics(Epic mode in wotlk). Just like normal instance gear (65+ instances) is what you aim for before you start them in tbc. So the ones who are raiding SWP will be able to tear through the expansion and maybe pick up a few blue pieces, but still can keep their t6/swp loot if they wish.
Also, the trinkets and other tidbits may last for a lot longer. Remember the BV/BR trinket from saph? it's lvl60 but still best in game. Theres a few other old Naxx loot that is better than even MH/BT/SWP drops.
Same will be with TBC items, a few rare trinkets/armor pieces/weapons, even tho massively lower in Ilevel will be useful even at early/mid/endgame raiding.
Also, blizzard didn't lie... the "gear reset" was not that bad, since most are rocking in atleast a few badge/pvp pieces, if not raid gear at the end of TBC.
Pre-tbc... how many guilds in reality, did reach Naxx? How many even got C'thun or Nefarion down? Everyone was a lot less geared so the jump was huge for most. Now it's not that huge
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