If your wife - or any other rider - favors a certain bike for a certain task, I'm all for it. But small wheel bikes being particularly favorable for aging riders? I'm not buying it.
Lest you think I'm a big wheel bigot, I've been riding my Alex Moulton AM since 1984. I currently have and ride regularly a bikefriday tikit and a Dahon Curl. Folks coming from big wheels bike often comment that small wheelers' handling is 'frisky' or even 'flighty', but I've ridden small wheels for so long that big wheel bikes feel odd to me.
However, an upright bike's bottom bracket has to be at a certain height to mitigate pedal strike, and therefore the saddle will need to be at a certain height for leg extension...and therefore the rider will be pretty nearly the same height off the ground independent of the wheel size. And this leads us to ---> Center of gravity isn't meaningfully lower and it's just as far to fall.
Swing your leg over? Most - but not all - small wheel bikes have dropped 'top tubes', but those are still significantly higher than bikes like the
Sun Streamway.
Going around potholes? Yeah, I'll take small wheels (frisky!) I
gotta go through a nasty pothole? I'll take a
Truebike.
I hope to ride my Moulton for 50+ years, but I've thought about what comes after. If it comes to a time that I don't feel secure and safe, my current thinking is I'll migrate to a crank forward bike
where I sit lower and can put my feet down with my bottom in the saddle. Maybe a bike like a
Rans Fusion ST.
Some years beyond that, perhaps something
with three wheels and finally, hopefully, going out
recumbent.