Intel x86 looking like OpenSPARC CoolThreads; Single-Threaded Myth
Most software is already using multiple threads, from desktops to servers.
The last single-threaded OS was DOS with Windows 3.1... but there were even TSR's back then.
With the advent of Windows 95, spawn windows spawned new threads. Dozens of services require CPU time with NT. Menus are now being checked in the background for usage statistics & being being rebuilt reformed. Indexing is always happening in the background. Speech recognition leverages threading and lookups. People are playing music & video in the background on their PC's all the time. Video & Audio chatting is normal now, where those incoming and outgoing streams are constantly being compressed and uncompressed in the background. People use their computers while rip'ing CD's and burning CD's. People are using their computers while they are recording videos from their TV, as well!
People are always running with multiple applications on the screen at the same time, and those applications are normally running more than one thread.
Most servers heavily use multiple threads and processes. Often, there are hundreds of processes and thousands of threads (my 1 processor 6 core 24 threaded lightly loaded server runs 611 processes and 1786 threads, while my heavily loaded 8 processor 16 core sever runs 252 processes and 2568 threads.) Web services, databases, and applications leveraged by multiple users are heavily thread dependent.
More cores and threads allow these interactions to be more seamless, increase throughput, reduce overhead of context switches, better manage performance peaks during high user counts,
The single threaded operating system and application was a myth promoted by chip manufacturers who were behind-the-curve technologically... If it was not a myth, these chip-manufacturers would not have been investing trillions into multi-core and multi-threaded silicon.