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.

Performance of UI is mostly single threaded

Sure, but there are very few UIs that require 100% of a modern CPU core. Exceptions exist, but they're just that, exceptions.

I'd go one step further and say that if modern apps are doing that that they should move the CPU heavy tasks out of the message thread. (And most have, again, since we're like 10-15 years into multi-core systems being the default.)