The code base has changed quite a bit since 198x:

- https://blogs.msdn.microsoft.com/oldnewthing/20040525-00/?p=...

- https://blogs.msdn.microsoft.com/oldnewthing/20160628-00/?p=...

- https://blogs.msdn.microsoft.com/oldnewthing/20180704-00/?p=...

This might also explain the choice of C++. Rather than rewriting their calculation engine in another language (it works, so a rewrite is rarely a good idea), UWP (not WPF; XAML is merely an XML dialect for object graphs and works pretty much anywhere if you want it to) allows you to write the UI in C++ (C++/CX to be precise) as well, and the UI code is for the most part not that complicated that C++ is a major impediment. That, and of course, most developers for Windows applications at Microsoft are familiar and well-versed in C++. Rewriting it in JavaScript or C# may take longer.