The biggest cap on success is player skill- positioning, knowledge of combat mechanics and map layout…etc.
If you have a level 35+150 Kruber and have been playing cata for a hundred hours, you will be good enough for legend with any other character, even if you have never played them. Your skills translate and the playstyles of the classes are different but not vastly so. Leveling each character separately and having a power cap just forces good players to go back to difficulties that they find boring.
I would be happier if all characters were levelled simultaneously and the difficulties were gated by “complete X number of games on the lower difficulty to unlock the higher difficulty”. X should be a small number, like 5-10. People will naturally gravitate to the difficulty that they find most fun, where they are challenged without being constantly overwhelmed (being carried isn’t fun). The current system doesn’t stop bad players joining high difficulty games, it just forces good players to play at boring difficulties and grind for to reach some meaningless power gate.