Sure, I actually agree. But I'm also a) not American b) not black c) not an anarchist, nor a liberal [I'm a radical socialist by conviction, not a liberal]. So take following with that in mind:
There are two separate things here, to be analyzed separately: slogan/demand vs . an actual proposed policy. I think it's a huge mistake to confuse the two.
So I dunno, I don't walk around saying "that's the wrong demand", or "say something else, you don't want that." I'd rather understand where it's coming from and who is saying it, see?
Slogans are tactical, part of a movement, and the activists involved do not have state power, they pretty much never will given the structure of the US state, and as I see it the purpose of the demand is to primarily to point out the injustices, contradictions, and limits of the existing system.
In that respect "defund the police" is probably somewhat successful from the POV of the people who propagated it, who are mostly, from what I can deduce, anarchist movement types. (And yes, they probably actually do believe it. But I'm not an anarchist, so) They've made a broad swathe of the public see something that is frankly true from my POV and others: policing as we have it now has an awful history, and frankly its primary purpose in our western society is the routine maintenance of existing power structures and inequities, and in the US especially this takes a fundamentally racist form. It's getting increasingly hard to deny that, even for mainstream liberals and some conservatives.
Is it universally true? No. No phenomenon in society works that way. But on the whole there's a structural aspect of policing that may not be reformable in our current society. I saw a poll the other day that a very large % of black Canadians (my country) trust the government, but do not trust the police. I think that's an interesting and powerful distinction: Especially in light of how police have behaved here at e.g. various anti-vaccine/anti-COVID measure protests vs how they have behaved at other protests (BLM, etc.) in the past.
I think "defund the police" is a "transitional slogan" which might boil out into something better in terms of a more concrete demand if the movement it floats along in grows.
That's my take.