Re: "The only way to do serious computing - servers, supercomputers, science?"
The number of web servers out there easily dwarfs all these examples you mention combined. Hell, I could probably replace "web servers" with "apache servers" and it would still be true.
But lets say you're right and the banking sector has more servers than the entire internet. How many of these non-FOSS systems were built in the last 10 years?
Sure, there are lots of legacy systems out there that are not running on FOSS, and yes, most banking and whatnot is still probably not running on FOSS - because it's a legacy COBOL system built in the 1970s and running on a mainframe emulator, or still running OS/2 or VMS. But I'll bet you good money that the majority of new systems being built today are running on FOSS or at least using it.
You must be pretty ignorant of current (as in, this millenium) trends to not know this.