This is well thought out and raises some good points about the economics of maximizing hardware and features over time. Nonetheless, I deeply disagree, since I think you're missing a couple key variables in your analysis.
Replacing a computer is a pretty big hassle every time you do it - dig out backups, migrate stuff, get email, bookmarks, working directories set up, go fetch data off the old machine when you realize you forgot it, etc. And if you had a crash, you have to add the replacement time. So a good analogy is to AC - you want it on and working when you turn it on. Interruptions won't kill you, but they can be pretty aggravating on certain days. As such your analysis is leaving out the value of minimizing downtime. Someone's sensitivity to downtime does depend on how much they rely on a computer (i.e., do they have a desktop they can use as a backup), but it's still pretty rare to have a highly portable environment on any computer. So a reliable machine and warranty and/or retail option that turns your computer around quickly are more important to a lot of folks than maximizing hardware and features over time.
My wife happens to also not like her Mac, and I'm going to take a guess that my wife and the OPs wife have relatively similar desires - do the things they knew how to do in Windows quickly, and not worry about anything else about the computer. What she mainly wants it to do is always work. So I think what she needs is a reliable, not too feature-focused machine with a setup and a warranty that minimize downtime. Whether the easiest way to get to that is to install Windows on the existing Mac box or buy a Windows machine depends on lots of real-life details only the OP knows. I will probably take a stab at installing Windows on my wife's machine and see how that goes over.
Of my 2 favorite things about using a Mac, one is going to the retail store* and leaving a little while later with my machine ready to go (although you do have to spend a couple hours ignoring the condescending hipness of the "geniuses"). That only applies to problems that can be fixed by configuration or swapping out hardware, but that's a pretty good fraction of all problems - HD, PS, RAM, battery failures have to be O(50%) or more of hardware problems.
* My second favorite thing about the Mac is that sleeping and hibernate rarely causes crashes. You open the case and the computer is as it was before going to sleep, with some occasional confusion about network changes. That was just never the case in the Windows machines I had - I attribute it to being able to define the hardware and test the software and hardware together. I like other stuff about the Mac, but they're way behind those two.