That's significantly less true than it was in the past. I think it's pretty easy to argue that the browser is the most important desktop app now and all modern browsers are multiprocess / multithreaded.

Beyond that, almost everything that does signal processing of some shape or another (audio, video, whatever) is multithreaded these days, so that one's out too. Compiling code is multi-process too.

I actually struggle to think of important work loads that are single-threaded these days. Obviously most simple apps are, but mainly because they don't use enough CPU to bother optimizing. We're more than a decade into multi-core systems being the norm and most CPU-heavy apps have taken advantage of that.