The core difference is in our mindsets. As a melee main, I want to see more opportunities for skill differentials and optimization to exist within my chosen role. I think a lot of melee players are of a similar mindset and determination, as they just want to be challenged on their role and push themselves to the limits. They don't really pay much heed to whether other roles push themselves or not.

Casting is often cited as being a 'skill differential' on magical ranged DPS, although it's certainly not a unique feature of the role. It sounds like you don't enjoy the skill differential on magical ranged DPS at all, but you also want the skill differential removed from melee so that they don't show you up in front of everyone else. It's really amazing how different the mindsets of players on different roles can be. We are not the same.

I think if you collectively feel as a role that being forced to stop and cast is not enjoyable, then you should be discussing that topic instead of positionals. There are plenty of games that allow movement while casting. You'd functionally be playing a physical ranged job with a 5 second GCD. Far be it for any of us to try to change your gameplay preferences. We just ask that you offer us the same courtesy.

I have no idea how 'positionals limit boss design'. If a mechanic renders a positional temporarily inaccessible, players can always find workarounds, and they've introduced a new system by which they can swap between a 'positional' and 'no positional' mode for specific mechanics like the M4 soft enrage. Melee already have plenty of considerations around AoE mechanics already that intersect really nicely with positionals, so it's not a 'one or the other' situation. They work synergistically to challenge players on the role.

In short, there aren't any good reasons to remove the skill differentials from melee.