Oh, you're completely right that a wider spectrum is valuable. I do not question this.
My assertion is twofold.
First, 5e was designed with the theory that most things need to be tailor-made for the math-averse folks TwoSix described. That's a problem, because it can very easily turn off the folks who want more. (I'm sure someone will come along and give 3PP as a solution, and I've already said elsewhere why I find that not actually a solution.)
Second, that 5e has continued past editions' flawed, bad, damaging, rigid link between spells = complexity = flexibility (and thus power), while martial = simplicity = inflexibility (and thus weakness). There is no such thing as a simple spellcaster (I'm sure someone will come along and give Warlock as a counter-example, and no, the Warlock ABSOLUTELY IS NOT simple.)
If we're going to take the "a spectrum is needed" response seriously, there needs to be a spectrum across thematics, not just in the generic. There needs to be at least one spellcaster that is comparably simple to the Fighter, even if it's still a little more complex (e.g. Battle Master level). There needs to be at least one martial that is at least comparable to the Warlock or Sorcerer. And we really, really need to fix the whole "simple things are usually weak unless heavily optimized, complex things are usually very strong unless played incredibly casually" problem. 5e has taken baby steps on that front. There's a lot more that can be done.