Honestly, a better way to handle it would be a "level squish" for all xpacs.

Ex:

60-70 = 60-65
70-80 = 65-70
80-85 = 70-75
85-90 = 75-80
90-100 = 80-85
100-110 = 85-90
110-120 = 90-95

I doubt they would do this however, but they could do this, keep time "time sink" the same, and it would accomplish the secondary effect of lining up the "paired xpacs" with their relevant levels (BC & Wrath = 60-70, Cata & Panda = 70-80, WoD & Legion = 80-90).