Why is it assumed that this isn't a more attractive proposition compared the alternative. Certainly there would be many, if not most or all, that would be willing to make this choice.
The point you are implying is that "career success and personal goals minus children" > "making smart babies and being a mother"...where ">" can be more enjoyable or fulfilling. It's not clear that this is true. Part of "baby making" means leveraging that IQ in a productive way to raise strong, intelligent children, so it would not be going to waste.
He'd even make money, because... well, be honest, how much would you pay for that if it really worked?
I'm not sure why you're asking me this when your opening post in this exchange starts by noting low fertility amongst highly educated women. Smart women generally aren't short of offers from smart men to make babies with them (and the smart men are usually pretty wealthy). If "many, if not most or all" wanted to have lots and lots of kids instead of being academics or lawyers or rocket scientists, they'd already be doing it.
I don't think a creepy eugenics foundation that's obsessed with the IQ of their kids makes the proposition more attractive even if they're paying enough for a slightly bigger mansion and the husband to afford to retire too.
But Elon Musk is a little bit too rich to make his decisions on partners and kids based on whether some poorer billionaire offers a stipend. My point is upper middle classes intellectuals won't be interested trading those decisions for the money either
Seems to me that the upper middle class is motivated at least as much by prestige as they are by money, family, or anything else.