I don't have a good answer to that question, I'm not sure anyone does. But if society collapses, much of that knowledge goes with it. Our society is incredibly complex and fragile, with a very high degree of specialization. That makes us vulnerable to these kinds of issues.

While the "dark ages" were hardly dark and ignorant, the collapse of the Roman empire meant advanced road building and concrete were lost for 1500 years. So there is some historical precedent here.

500 years would put us back to 1500ish, I can’t see that, steam technology and electric motors/turbines are simply doable (and where done) with not much technology to boot strap from, hand tools made machines which made specialised machines which repeated for generations but the first step comes back to steam and electrification.

It’s not like every book in every library would or could disappear.

I think there is a problem with knowledge being very dencly concentrated and supply chains being so long.

If you took someone who worked in a steel mill I bet they would have no idea how to produce steel from first principles. They know what their job entails, they know which bits of equipment need serviced an maintained but they don't know how to get from rocks to bar stock.

Similarly if you had a perfectly working steel mill and you cut the supply of coke or iron ore few would know how to get it up and running again. If you cut the supply of the alloying elements you might get a passable mild steel but not one to the exact specifications you require.

I think it's probably surprising how few vital links you have to cut before life as we know it breaks down.

We might lose electronics and such, but we won't forget that a wood stove is more efficient than an open fireplace. We won't forget that it's a good idea to dig your outhouse far away from your source of drinking water.