Opening knowledge in chess is almost entirely unnecessary below a rather high level, perhaps 2200+.
The reason less experienced players get obsessed with openings is because they make regular tactical mistakes in the opening, and then blame lack of opening knowledge, as opposed to their lack of tactical ability. In other words they try to memorize their way out of tactical mistakes, which is impossible. At that point, after it inevitably fails to work, they claim they've plateaued, lack the IQ, maybe the memory, maybe are too old, or whatever other excuse.
This is made even worse by the fact that opening knowledge is ostensibly easy to improve whereas tactical vision seems very difficult to improve. In reality it's the exact opposite. Exact opening lines will fade from your mind rapidly (though general ideas will stay with you forever - but that's another topic), whereas grinding tactics might not 'feel' like you've learned anything, but overtime will permanently train your intuition to where it needs to be to start seeing major gains.
On top of all of this - one could simply play 'freestyle' chess (the starting pieces locations are randomized) and suddenly there is 0 opening theory. But you'll find that your freestyle rating is going to be strongly proportional to your 'normal' chess rating!
I think it's imprecise to say that opening knowledge is unnecessary. What is unnecessary is opening theory, or more specifically, rote memorisation of opening lines.
This is different from opening understanding. Understanding the importance of tempo, development, controlling the centre, the different pawn structures, middle games and endgames that result from different openings, the plans and motifs typical in various opening complexes. Any late beginner to intermediate player needs to pick and study an opening. The problem is that instead of studying the opening, players try to memorise lines without improving their understanding of the resulting middlegames, and the plans they should be playing for. Then, when their opponent diverges from the main lines(which in my experience happens in 99% of games between players below 2000, because it's very rare that both players have memorised the same long line), they don't know what to do.
I'm a 1900 FIDE player, I have an opening repertoire of sorts. For instance I play the modern benoni with black. An extremely theoretical opening, and yet I have only a small handful of longer lines actually memorised, because they're simply too complex for me to figure out over the board(e.g the b5 lines against Bd3 h3 Nf3 setups). But what I have studied extensively is the strategic landscape of the benoni, games by strong players in the opening, etc. And I have years of experience playing the opening. I know what kind of exchanges typically favour me, or my opponent, what pawn breaks each player should be trying for. And all of that knowledge is crucial for me to get anything out of the opening. I have beaten players tactically much stronger than me in this opening simply because my understanding of this specific opening was better than theirs.
Tactical ability is obviously important, but it's definitely not everything.
In general I certainly wouldn't disagree with this, it's what I was alluding to with general ideas that stick with you. But I'd call this a different thing than opening study. For instance one can get Benoni like structures in the King's Indian, Benko, English, Nimzo, and more! And so it's not really understanding the opening, but understanding how to play a certain structure that arises in many different openings.
And it has nothing to do with memorization. I mean you mentioned the b5 stuff against Bd3/h3/Nf3 setups. You might not be able to calculate the depth of what happens if white manages to hold onto his extra pawn, but you can certainly calculate to at least the point of 'okay, I'm getting my pawn back in most lines, disrupting his center, and getting my play going. if the one line where he holds onto it (Bxb5 stuff) then he's going to have a bit of difficulty castling, his pieces look disorganized, his extra pawn and b2 both look weak.' That's more than enough on general principles to go for the sac I think.