Do Baseboard heaters become less efficient overtime? Should they be replaced before
Failure or left and changed at failure?

I've had them in 3 different houses over 30+ years and I have never had one go bad. That doesn't mean they won't go bad but that is my experience. I have had some rust in a couple of instances, but I just repainted them with high temp paint.

I've seen many go bad. From my experience they do not degrade in performance, they just die.

Thanks guys, it seems your answers confirm my own beliefs that they are either working or not.