The development team doesn’t seem to be self consistent in their reasoning. If good pet charms are only available from “current” content because we should only be doing “current” content, then “old” content shouldn’t be better than “current” content.
That said, I don’t think that pet battle content should be considered “old” and “current” in the same way as raiding. Pets stop at level 25 no matter if caught in Westfall or Stormsong Valley. Pets considered particularly good at various battling roles are found across different levels of content.
However, rewards really inconsistent across content for their value. A “current” world quest might be for a single boss and award war resources (experience not worthwhile, nothing in stuff of value for pet battling). Going and doing a MoP daily will award experience and a bag with a chance of a rare stone/bandages (more useful than the “current” world quest).
Since “old” pet content is pontentially more useful than “current” pet content, could we use our shiny charms to buy the generic “plus one level stone” and the generic “upgrade to rare stone” at some price that reflects the increased availability of the shiny pet charms to the polished pet charms?