You don't need to wait 20 years for this. Often it's weeks or even days.

That implies that Hacker News will be around in 20 years. Newsweek will be (in some format or another).

I genuinely thought that twitter was a ridiculous fad for trend-whores and it would die quickly, or remain a tiny niche. I've been wrong about tech so many times personally it's ridiculous.

At the very least, I think it's pretty safe to assume that in 20 years the Internet Archive will still be around and we'll be able to look at 20 year old HN threads there.