Question: Does granting range players the ability to retain more and more damage on the move (as illustrated by the upcoming Lightning Bolt changes in 5.3) make it harder and harder to design encounters where melee DPS are not trailing behind?
Answer: Traditionally, the melee advantage was being able to do damage while moving, but now a lot of ranged are also good at movement. We could certainly go back in and prune a lot of cast-on-the-move and instant spells from casters, but on the other hand we know players think those abilities are fun and you can even argue that having to stand and “turret” as a combat mechanic feels a little dated. There is also a continuum here: casting Lightning Bolt while moving isn’t a big balance problem, but something like passive Kil’jaeden’s Cunning might be.
Rather than making casters terrible at moving, we’d rather develop a niche that melee are really good at. For example, we could emphasize that melee are really good at cleaving multiple targets, or they could be more survivable, or both. We are going to explore these ideas more.
Looks like they've changed their philosophy on penalizing ranged for moving. Figured they were going in a different direction when I saw that Lightning Bolt patch note. Personally I felt like they weren't delivering on that design anyway all throughout the expansion, and like they touched upon, not being able to cast while moving isn't fun. I wasn't a fan.
I hope this opens the door for Frost and Arcane getting better mobile DPS, whether it be via a Spiritwalker's Grace-type ability or something else, especially if they're going to let other ranged do great mobile damage. Boss fights are generally becoming more movement heavy over time. In a vacuum Arcane and to a lesser extent Frost might look fine but they really are just an outright inferior option compared to other ranged in mobile situations.