re: Do Wifi routers "wear out" over time?

Lol, you aren’t going to see noticeable performance degradation in parts. You will see failure or poor software/firmware performance if you even upgrade your routers firmware. More likely performance issues will be on the computer side of the connection with whatever garbage you been putting on the computer.

Best Buy guy is just trying to sell you shite cause that is his job. They will lie to make the sale. I did when I worked there decades ago. I lied about monster cables all the time. I felt bad about it.