Hardware

I also suspect that a USB-connected DAC is going to have a longer life due to interface longevity.

An old PCI sound card can't be stuck in a PCIe slot. An old ISA sound card

and I'm pretty sure I was using one of those when USB was around

can't be stuck in an PCI slot. You can probably find some combination of cards-on-cards that will work, but it's not a straightforwards "plug it in and use it".

You can still use original USB devices on current computers. At some point, computers will probably stop having USB-A and you'll probably need to physically adapt USB-C to it, but the electrical backwards compatibility is still around, and I doubt is going to end any time soon.

Firstly, motherboards for desktop PCs don’t have terrible onboard sound like they might have in the old days.

So, you want to not have electrical noise from your computer spilling into your audio output, true enough. I've had that in the past from on-motherboard DACs, true. But USB power is also often dirty as all hell

I remember watching some YouTube video of someone running around with an oscilloscope and showing all the stuff that shows up on USB power. It was appalling. I've had noise from USB power spill into external USB DACs that don't have adequate internal power supplies running the analog circuitry to deal with noise from USB power making it to the analog signal. This inexpensive thing definitely permitted it through, tossed one out the other day (though I've also had higher-end USB DACs on the same bus that didn't have issues).

I have, in the past, kind of wished that people reviewing USB DACs would have some mechanism to induce noise on USB power and measure the degree to which it is permitted to leak through into the analog output.