I think the key thing here is that you are running ancient hardware. It has been hard to buy PCI devices for the last 6-7 years
This is simply untrue.
Newegg *still* lists twice as many PCI sound cards (43) as PCIe ones (21), 37 different PCI-based USB add-in cards, and 13 different PCI-based hard drive controllers. Heck, they even still list 36 different PCI-based video cards (though nothing with a current-generation GPU, obviously).
and even as far back as 2008 it was rare to find a board with more than one legacy PCI slot, included to appeal to the fringe cases where people stubbornly refused to update one old card.
Also untrue. I own a dozen or more motherboards from after 2008, and *all* of them have at least 2 PCI slots. Asus M3A78-CM (2009), Asus M5A97 (2011), Asus M5A97 R2.0 (2012), M5A78L-M/USB3 (2013), and many more. None of them were selected specifically for the presence of 2+ PCI slots, as the only PCI card I own that I might potentially want to use is an old Turtle Beach soundcard (great ADCs on that thing...)
Now, don't get me wrong; I agree that except for a few unusual corner cases, it makes no sense to buy new PCI-based peripherals these days. But a lot of people still have 'em, and replacing a working card just because it has an "old school" PCI interface doesn't make much sense either, unless it is doing something which would benefit from the extra bandwidth.
Sure, your PCI cards may still work, but I have a dot-matrix printer that still works - it doesn't mean its a good idea to still use it.
Apples and oranges. Dot matrix printers suck unless you need to print multi-part forms (which require an impact printer) or do a lot of printing where output quality doesn't matter (dot matrix will save you a lot of money on consumables in this case). They have a narrow set of things they do well, and if you don't happen to fall into one of those use cases they make no sense.
On the other hand, a PCI-based soundcard or SCSI scanner adapter still works just as well as a PCIe one (assuming you can get drivers for it that work with your current OS).