[Daisuke]

That's the entire point of a mage though
Mages are versatile casters with a bunch of spells
All a Void Assassin does is get behind you and cut your throat

That's like saying "Evokers shouldn't be the only ones able to hit really high numbers even though they're a Nuke Mage and this is their concept".
Look at MG. Versatile with various shells that have different effects. Scales with will.
Monk. Various different techniques that hit certain elements, you know what setting sun and aid scales with?
Verglas, Different techniques with different playstyles that can be chained into each other. Will scaling and will growth.
All of those can play very differently despite being the same class.

Now look at BK, Ghost, Kensei, VA.
They're very straightforward and it's generally going to be their innates and stats that decide how effective they are. You can't be a BK and expect to hit elemental weaknesses, scale with almost every stat, or switch between onhit effects.
Does this take away from their core identity or have a large impact on how they play? Not in the slightest. Say a BK doesn't get to equip all their skills. Do they stop wearing heavy armor, having tons of defense and crit evade, and having low amounts of offensive options? Do kenseis stop being a crit/dodge machine because of their innates? Does ghost not get stronger as they go lower in HP? Will a VA not try to get behind you for the flank bonus?

That's what I'm talking about.