That's a really good point. Most slang is ephemeral and quickly becomes dated, but occasionally a term will successfully enter the language on a long-term basis. I used "cool" recently as an example of one such term. "Okay" is another one, although most people now don't realize it because its origins predate us.

There's a demonstrated phenomenon that people tend to continue to use at least some of the slang that was current during their teen years well into their adult lives. This is also going to contribute to the continuing usage of "cool".