I think the gap is the question of how many people there are who want enterprise-style lifetimes but don't actually want support. If you're running servers which don't need a paid support contract, upgrading Debian every 5 years is hardly a significant burden (and balanced by not having to routinely backport packages). There's some benefit to, say, being able to develop skills an employer is looking for but that's not a huge pool of users.

I think this is the reason behind the present situation: CentOS' main appeal was to people who don't want to pay for RHEL, and not enough of those people contribute to support a community. That lead to the sale to Red Hat in the first place and it's unclear to me that anyone else could be more successful with the same pitch.