My frame of reference for the argument about Mesos vs k8s is from earlier in the timeline when the k8s was up and coming and just starting to compete with Mesos. Just as in my analogy with Rails: at it's inception, it lacked a bunch of features, but it was accessible. Years later, Rails had many of those features.

k8s is the same. Operators didn't come around until late 2016 by CoreOS and even then they weren't widely adopted. It wasn't until after RedHat bought CoreOS and pushed operators as a pattern into k8s that the pattern took off. As it is, the k8s version of the operator framework only went 1.0 last year.

Finally, I should have pointed it out in my original comment, the Mesos vs k8s comparison isn't perfect apples and apples. Mesos is just a component in a stack, whereas k8s is effectively a full stack. Again, Rails was pitched as an opinionated, batteries-included framework compared to many of the more focused frameworks that, for their niche, may have been better, but the convenience of having everything in one packages won out.