With the proliferation of spell interrupts

With the proliferation of spell interrupts and other control mechanisms, and the ever-increasing value of mobility, how do the developers plan to successfully move casters away from instant casts and make hard casting in PvP more viable?

More detail:
It was recently stated by a po world of Warcraft CM that the Developers would like to move away from instant cast spells, and back towards hard casting. This is a difficult philosophy to comprehend since the specs that rely on hard casting, in Season 9, have had the most difficult time succeeding in PvP. It seems improbable, with the game in its current state of design, that we can bring back hard casting viability without massive structural changes.

Edited for clarity/brevity.