#1 Isn't that a waste of wallet address keys?
That makes no sense.
#2 Do they have an expiry date?
Does what have an expiry date?
#3 Can they be destroyed? So someone else can use the key?
You would never, ever want to use a key that someone else ever had access to.
#4 Would we have the same problem as IPv4 in future where there is a payment address key scarcity?
The security properties of the bitcoin system render all these issues almost literally meaningless.
Either there exists one or more locks that a key opens or not. If such locks exist, then you still need the key and are still using it. If no such locks exist, then the key is just a piece of data sitting on your computer -- it has no effect whatsoever on anyone else.