A cool URI is one which does not change
What sorts of URI change?
URIs don't change: people change them.
There are no reasons at all in theory for people to change URIs (or stop maintaining documents), but millions of reasons in practice.
In theory, the domain name space owner owns the domain name space and therefore all URIs in it. Except insolvency, nothing prevents the domain name owner from keeping the name. And in theory the URI space under your domain name…