This isn't a QoL suggestion. This isn't even close to what qualifies as a quality of life change.

I don't see the point to this at all. You can cancel your cars and instant cast spells still have you waiting out the GCD. Though you can act, so healers woof become insanely mobile. However making all DPS spells instant would literally break WHM. Mobile, instant cast Holies, mobile instant cast Aero 3. Just insane