The type of person who desperately clings to his or her creaky old ps/2 mouse and ps/2 keyboard probably isn't going to spend a lot of money on computer hardware anyway.
You might be surprised how much money us "PS2" folks spend on equipment ;p
When I stop having clients, and reading threads in forums, that say "my USB keyboard doesn't work right", "my usb mouse is acting funny but my PS2 one works", "is anyone else having driver problems"(for a keyboard????) then maybe I'll stop using PS2.
USB was a port design pushed by Microsoft and its partners on an agenda for a media PC. It was intended to be the answer to Apple's use of IEEE-1394 (aka Firewire). Unfortunately, USB is far inferior when it comes to data transfer rates and maturity. It is also crippled with a poorly designed connector that is easily knocked out of contact (Firewire also suffers form this to a great extent), and a maximum usable cable length that (without powered extender/hub) can be ludicrous.
Contrary to popular belief there are also people out there that use their systems for more than surfing the net and playing games. Much scientific, enigeering, test equipment and numerous other pieces of technical equipment that is computer controled relies on RS-232 serial and there is a huge data acquisition market (as well as high end laser printers) that depends on parallel interfaces.
There is good reason that high-end work stations still have "legacy" ports. And before USB-RS232/Parallel converters are mentioned - they don't work properly with all equipment/software packages.
Computers are a tool, like anything else; buy the one that has the features you need for the job you need to do - but be aware that not everyone has the same needs.
As regards "legacy" technology, some motherboard makers have even started putting a ISA slots back on their motherboards (for example http://news.thomasnet.com/fullstory/487906