Now, personally, I think alignment causes problems due to its contentiousness, and should be removed for that reason. And since, as OP has observed, D&D doesn't need it, I don't see much in the way of good reasons for it to continue existing in the game. The main one seems to be tradition, and a secondary one seems to be "a shorthand." I don't care about the former and I think the latter could be satisfied with something less contentious.
Perhaps there is an important distinction to draw here.
I value "evil" (whether capitalized or not) as a descriptive tool for narration and story construction. E.g., I can tell my players that the Cult of the Burning Eye is "bad guys" who do "evil" things, which they can then confirm for themselves if they wish. I can tell them that demons are all evil by choice (having decided that breaking things and messing stuff up during the War in Heaven was more fun/interesting/worthwhile than
either of the proper "factions" of the war), and yet still have a redeemed succubus* who is not evil.
I do not much value "evil" as an expressed mechanical component of the rules.
That almost always invites the real controversy, e.g. "all necromancy spells are evil" when that includes things like "spell to keep my buddy's body from rotting so we can get a resurrection." Such prescriptive diktats are not just often badly-reasoned, they're usually broken by the very ruleset promulgating them. Such things mostly just waste my time, confusing issues that should instead be
clarified by the use of words like these.
Saying, "Is evil necessary?" strongly communicates to me that it should be done away with entirely--that one should never speak the word in the context of D&D at all, and anything that might do so should be scrubbed of it. I mean, is there any
other reason to talk about "unnecessary" things? We live in a time where the game design fashion is minimalism, and the "necessity," or rather lack thereof, of a given element, whether in rules or descriptions, is almost always treated as its justification for inclusion vs exclusion.
*Rather,
ex-succubus, since she actually managed (without realizing it) to change her true name. During the War in Heaven, she was Al-Yattara. After various journeys and getting a legitimate shot at changing her ways, she found she actually really loved the mortal world, because it was full of so many beautiful
things, things that would be lost in the cacophany of her unending hunger if she fed it. Then she met a human, Badr, that legitimately charmed
her, and they raised a family together, though that was long ago. Her great-grandson is the party bard, who helped her realize that her husband is the one who gave her her new name,
Zamira. She was his nightingale as he was her moon.