Why would blizzard change the leveling content ? It's been the way it is for 10 years, it got some revamps with cataclysm and been left alone again ever since.
Altering the comprehensive leveling content (which includes all the past expansions) now would cost them a fortune in development time for literally zero gain.
If anything the biggest change that could reduce the quality of leveling content is to speed up XP gain so people level through zones much faster and this has already been done for all pre-cata content but that change, if anything, actually reduces the number of people who will buy an extra boost by reducing how much time and effort leveling actually takes.
When I started playing in Wrath it took me over 3 months of playing several hours a day, almost every day, to get my first character to level 80.
Now I have several characters at maxlevel and several quite close to it - you can do it in a few weeks, or with sufficient dedication (and heirlooms and boosts) one long, hard weekend without sleep.