The five year reign of RHEL7 was too long and did result in the very issues you bring up, but the ~3 year duration of RHEL 5,6 & 8 was short enough to avoid problems due to attrition in enterprise settings (unlike startups which have higher turnover, and not counting bus factors of one - no release cycle can't solve that).
And like others have pointed out, automation doesn't help as much when moving between releases. We have everything configuration controlled with kickstart and ansible and/or docker, and it is great for reproducibility within a release cycle, but it doesn't save much time or knowledge between releases. And Ubuntu is even worse in that regard despite having a shorter release cycle.