I don't think you really have any idea what you're talking about, to be honest - there are millions of RHEL 7 and CentOS 7 boxes in production right now, and many of them are serving websites and could benefit from SSL certificates.
I know of a couple large organizations that have tens of thousands of RHEL 7 machines that use letsencrypt for customer websites, and I sent this project to sysadmins on their teams, so it's very realistic to expect that businesses are still using supported software.
Not everyone can just upgrade to the latest version of Ubuntu every 6 months for their production workloads like you might expect.
Yes, but which version?