"Of course, this would seem to suggest that you assume that evolutionary releases are introducing more bugs than they fix with each successive release"
That is indeed possible but not what I am suggesting. What I know instead is that even evolutionary updates introduce *new and different* bugs from the previous version while introducing new features and fixing some old bugs. Red Hat cannot simply just use the latest upstream user space for major codebases like a desktop environment or a office suite as has been suggested here. QA, hardware and software certifications take a lot of time and effort.
This is not to claim that backporting is a panacea but it is a solution that has worked better than anything else for the most part for the target market and Red Hat does update leaf packages like Firefox more often and introduce parallel installable versions of new software like Postgres. In the case of Postgres, some customers want the new features in it but simply upgrading to the latest version is not a option when the new version also introduces incompatibilities.
If more upstream projects would stop introducing so many (often avoidable) incompatibilties and take API/ ABI stability more seriously, it would certainly help but I am not sure there is a definitive movement towards that.