Even unpaid deployments create an ecosystem of skills, courses, books,
forums, blogs, etc - this is valuable to the paid-for product as long as
there's still a 'conversion rate' from free to paid-for that is high enough
(in absolute numbers) for the paid-for product provider to make a profit.
In some ways CentOS is a perpetual 'free trial' for people who are more cost
sensitive or want to deploy a new system very quickly, vs. people who need
commercial support - when some CentOS users get more budget, maybe in a
different company or somewhere that rapid support is worth paying for,
they'll go RHEL. If CentOS was very different to RHEL this switch would be
harder, and ultimately CentOS would be less valuable to Red Hat.