I don't remember discussing either of those while writing Qt.
Rather, single-threading followed from two big points. First, the user calls the program rather than the other way around, and the user isn't multithreaded. Second, there aren't performance problems with the UI, and certainly none that require fighting #1.
Some programs need more than one thread. But that need does not originate within the UI, and complicating the UI for it would comply with RFC 1925 point 5.
Rather, single-threading followed from two big points. First, the user calls the program rather than the other way around, and the user isn't multithreaded. Second, there aren't performance problems with the UI, and certainly none that require fighting #1.
Some programs need more than one thread. But that need does not originate within the UI, and complicating the UI for it would comply with RFC 1925 point 5.