TheCosmicKid
Hero
Even if there were only two practitioners of arcane magic in the entire world, two distinct lines of progression would be justifiable. The classes in the book are abstractions: generalizations and sample platters of the sorts of things that heroic characters can learn over their careers, presented to us as discrete progression paths for fun and ease of use. In universe, unless all the wizards go to the same school and follow exactly the same curriculum (which, to be fair, does make for a rather successful children's novel series, I hear), they are probably going to be picking up more, fewer, or different side skills -- what we would call class features -- alongside the basic spell advancement. Hell, even in the book, I think you'll find if you look that there are already eight distinct lines of progression.How many practitioners of arcane magic are running around your setting, that it would make sense to have two distinct lines of progression for them, rather than just saying that the better wizards are higher level?
Back in the day, when the wizard class simply was the basic spell advancement, what you say would make a lot more sense. But now in 5E, with each class being a big bundle of features rather than just one or two that scale up over the levels, it's perfectly reasonable to imagine characters who have put different things in their bundles. A PC rogue isn't just Sneak Attack, he's also got Thieves' Cant and Cunning Action and Uncanny Dodge and so on. But I don't think you'll get far trying to argue that any character who can stab backs with a certain proficiency must also, by necessity, have a knack for rolling with blows to avoid serious harm.