This isn't true. Your biggest problem is that simple tasks like Safari, Mail, Twitter and Facebook take up significant storage (as a % of total space available on a 16gb model) over time.
I had 16gb with the 5 and 6, and while I managed with the 5, five months ago my 6 was full and ever since then I've been deleting/backing up constantly, meaning almost daily. I bought my phone on launch day.
It got so annoying that I dumped all my pics and deleted all my messages, and deleted and reinstalled all apps and Mail, and I was able to free up 4gb. So for the past three months I'vr been purposely avoiding taking lots of pics and deleting messages as soon as I can. It's not a fun experience.
I'm a similar user as your wife--I use a few basic apps and text. I barely even take pictures. But 16gb (11 usable after OS) was not enough for my 6, so it definitely won't be enough for the 6s, which is why I got 64gb. I'll never use the space, but peace of mind is more important to me. Yes, you can use iCloud or other cloud storage, but it's still more work than I'd like it to be. I want my phone experience to be the opposite of a computer--painless and without any fussing around.