Why wouldn't people just upgrade the heirloom when they're ready to actually use it? Why would someone pre-upgrade all of their heirlooms and not touch them until 9.0? I only upgraded my heirlooms when I planned to level an alt that needed to use them.
It makes no sense to spend hundreds of thousands of gold to upgrade heirlooms to level in an expansion that hasn't released yet, nor do we have all of the details in. Sorry, that's just silly.
If the goal was to sell more boosts, a level squish and overall increase in leveling speed would be pretty counter-productive to that goal, wouldn't you think?