Your Mists of Pandaria concerns are legendary.
Some procs include a stat-percentage instead of rating, so they would always remain BiS.
Blizzard fixed this by lowering the proc-chance if you level up.
If you have a level 85-legendary, you can be sure that it will be useless and that it will hardly proc at level 90.
now that you reminded me I remember using Bone-Link Fetish for the first 2 weeks in mop till they realized how well it scaled with weapon damage and it took a nerfbat . I hope they don't get them too low so at least they can be used for leveling till the next cap
I come across a quiet river, that wonders through the trees.
I stare into its running waters and fall unto my knees.
In resignation to the forest, that's held me for so long.
I close my eyes and drift away into nature's evensong.
Usually they remain viable for the equivalent of 1/3 of the leveling experience. For the metas, they just change them to not be socketable in items above ilvl (whatever warforged heroic equipment is)+8 (to account for upgrades). For the others, they change the procs to reduce over time as you level, so they're useless by the time you reach new cap. For the flat% boost trinkets, they change them so they don't function for characters above level 90 (there's item that have had this happen to them in the past).
They're fairly easy to fix using their existing methodologies.