Well, of course most women had babies perfectly well before widespread scanning. But the perinatal infant death rate was about 5x what it is now.
Placenta praevia isn't that uncommon, and a major one is likely to cause death of the baby and can cause the mother to bleed out too.
A footling breach or other position undeliverable vaginally is better discovered when there is still time to plan a section rather than have transfer of an exhausted labouring woman for an emergency one.
Some anomalies can be diagnosed in utero, and if specialist intervention indicated shortly after birth, then delivery can be in a hospital which offers those services (rather than blue-lighting a new born to nearest place which might).
Some conditions can even be treated in utero.
Or if something disastrous is found, then the parents have a chance to make active decisions on whether to continue and let nature take its course, or whether to intervene by termination.
Now, none of that applies to most babies But you cannot predict which babies might have problems, nor how fixable they are.
The likelihood may low, but the (possibly avoidable) consequences are huge. Has she really, imaginatively, thought what it would be like to live with the consequences should there be permanent averse consequences for want of a scan? If she has, and is still at peace with her decision, then you won't be able to persuade her and it is her choice.