Does Mac power adapter die if you leave it plugged in?
No.  You don't do any damage to a power adapter by leaving it in.

The fact of the matter is that power adapters die for one reason or another. 

Personally, I have power adapters for products that have been in use for well over 15 years, being plugged in all day, every day.  I also have power adapters that barely last a year.  For the most part, my devices that have these types are power supplies are plugged into surge suppressors with most plugged into UPSs; so I couldn't protect them any more than they already are.

In your case, it appear that the power adapter was defective and Apple took care of it under warranty.  Just keep on keeping on; no need to worry about leaving it plugged in.