The problem is, if the new content items aren't better than the old content items, it would be terribly disappointing. Yes, people expect to find a better <insert item here> in the next tier, otherwise there will be complaints of poor itemization and so on. If all of WotLK was the same ilvl, and you got some bracers in naxx that were the "correct" bracers for you, would that feel right? Or if they wouldn't drop, would you want to keep farming naxx until you get them?
Trinkets are the interesting exception, where there are some trinkets from previous tiers that still retain the top slots. But even if the BiS trinket for some class was in Uld, good luck getting your guild to go back for that. In that case, we're back to the
old days, where you have to join a pug and hope you win a roll against random alts in greens.
No thanks, I prefer upgrades in each tier of content. There can be interesting gear choices even among several items at the same tier; for example,
tier piece vs.
crafted piece vs.
drop.