No doubt, there's plenty of Gamertags that are not being used on the Xbox Live service that someone wants. Recently, Microsoft put back close to a million of them that were first created on the original Xbox and were never migrated. Now, anyone generated and not used in a five year time span will also be put back into circulation.
Given that if you haven't logged into Xbox Live in five years, it's pretty safe to assume you're not going to be using it again anytime soon. This gives folks a large gap of time in order to keep their Gamertag should they decide to wait a bit on picking up a new Xbox or logging into Xbox Live on their Windows 10 machine.