I'm not sure that argument actually holds much weight ..
Our current 24 limit doesn't apply in the car off a fetal abnormality anyway. 24 weeks is essentially there limit on social ground. If a disability is discovered at 30 weeks, the woman can still have an abortion. I know someone who discovered their baby was severely ill at 38 weeks and advised to terminate.
Changing the social limit to 16 weeks, 20 weeks or whatever will have ZERO implications on the ability to abort in case of abnormality.
There are arguments for an against it, but this isnt one of them.
Ps: The earliest babies to survive have been born at 21 weeks (I can't remember how many days). Babies can be born alive much earlier than this, but die shortly after. I don't know if anyone has tried to keep these babies alive, or if they receive palliative care only (which is what they'd get in the UK, but it wouldn't surprise me if not aggressive efforts were made in the USA on occasion).
I don't think we know what the earliest age we can keep a baby alive is, unless it was routinely attempted earlier, which would throw up all sorts of ethical and proportionality issues.