I would prefer if they were not linked to zone levels at all. I like to take my low level toons to higher levels. I understand that I do so at my own risk. Trying new ways to level is fun. I raised my baby rogue on Timeless Isle from level 20 up to max level. It was fun! I raised a toon herbing in Suramar (until they ruined that) and it was fun. I raised a baby toon lvl 1 pet battling in the Vally of Four Winds, and it was fun (until they ruined that).
Experimenting on different methods of leveling is dangerous, but it is fun! Making it so my little toons can’t get to these zones removes that fun.