The armor kits/spellthreads were designed to be relevant in MoP, and they're just stuck with level limits appropriate to the expansion they came from. I disagree and think it'd be nice if they were more accessible, but given that it's not applicable to max level and that the majority of the people it is applicable to don't pay, I don't see changing that being a high priority on Blizzard's part.
There are embossments from panda leatherworking (draconic/heavy/primal) that can be used to enchant your own pants, and that persist even if you change professions. Most players will have done this after having picked up whatever pants they see themselves liking. Note that while this doesn't require you to be heavily invested in panda leatherworking to do, it has to be done while you have the LW profession and consequently will require you to drop another profession if you want to enchant another pair of pants.
Intellect users can get something similar out of tailoring, although again the versions that can be stockpiled or traded are too high level for you and in this case the self-applied ones stop working if you drop tailoring as a profession.