"Moral" doesn't mean "based on whimsy". If you believe blood transfusions are immoral and try to stop them, the rest of us get to say, "Oh no you don't--there's great evidence of help, no evidence of harm, and your claim that it is a moral issue is not supported by the technical details."

If someone says, "it's immoral to cut your lawn because it murders moon fairies" or "you can't take my photo while booking me for a crime because it will leave my soul vulnerable to voodoo" or whatever, the rest of us say: no, that does not comport with reality. You may have a moral feeling about what's happening, but if you can't point to some real thing that's widely accepted as a moral concern, it's not in any important sense a "moral question". It is still a question of individual determination. If they don't want to cut their lawn, let's not make them. But in order for society to treat it as moral, it's got to be based in reality.

So, back to 15 vs 24 weeks. 15 weeks isn't based in anything. 24 is. So 15 is clearly wrong. Now, if you want to argue for 21 vs 24, because you worry that the range of neural developmental speeds is enough so there may be some 21 week olds who have the potential for close enough to a cognitive state that we want to count it, okay, sure, argue the science.

But the point is, it's got to be about science here, too. You can't just make stuff up, and expect other people to go, "Oh, because you have your deeply felt fantasy and color it 'moral', you get to tell me what to do!"

Personally, I'm not very comfortable either with "some distinction is needed and it's easier to draw a sharp boundary than do it case-by-case". But if you're going to do that, the sharp boundary ought to have some basis in reality. For instance, you could argue for legal adulthood anywhere between around 12 and 30 depending on what you focused on (e.g. possibility of reproductive maturity, full myelination of long-range connections to/from the prefrontal cortex, etc.), but neither 5 nor 40 would make much sense.

So, if someone wants to argue that a 15 week old fetus is meaningfully a person, go for it.

If someone wants to put the boundary there because that's what their conscience tells them in the absence of information, well, too bad. Get some information and try again.

(P.S. by "blatantly illegal"--obviously not a legal term--I meant something that wouldn't be overlooked as not worth pursuing, and is clearly well outside any fuzzy boundaries regarding the law's interpretation.)