Few want to pay for the knowledge, quality of the old school parts store.
These guys aren't gone because people stopped buying from them - these guys are gone, because they've aged-out and retired.
My dad managed a NAPA store for 30 some years, but, ultimately, when a man nears 70, he doesn't want to stand at a counter all day, slinging parts.
Couple that with **** management that doesn't want to pay people what they're worth, and you end up recruiting people barely qualified to run the fry station at McDonald's.
Finally, add to that a parts store that simply doesn't want to stock anything other than oil filters and adhesive-backed autobody mods, and you have a huge problem.
I've tried to maintain my parts purchases locally, but when every single shop tells me "I don't have it, but I could have it here in a day or two" - well, I can have it here in a day or two myself, save significantly over your price, and ultimately, get a better quality (sometimes OE) part out of the deal? Why am I involving a middle-man?
I'd point out that the lack of in-stock availability and stores stocking tack-on junk instead of parts was happening well before the huge boom in Internet parts availability. This nonsense was what helped drive me to shop online.
Don't blame **** parts stores on the Internet economy. Blame **** parts stores on **** management/owners who don't care to hire/retain quality parts people, who decide to move on to greener pastures.