I have used Mesos/Marathon from nearly the beginning and K8s. We currently use K8s. In your third paragraph about the 11 person team with custom scripts is why K8s has won out. It wasn't really the marketing by Google, it was that K8s provided by default what pretty much everyone needs (Discovery and storage management). Mesos took the approach that big shops like where everything can be extended and call APIs but if you are not very huge scale you really do not want to be writing scripts or doing any of that. You want an opinionated full stack.
Scaling from one node to 100s is not trivial in almost every stack. I just think that the requirement that something like that is trivial does not fit the use cases of almost everyone. There's always the "Can this be web scale?" question. We are able to add nodes on the fly but we would certainly face other non-k8s scaling issues with databases that are more complex.
And as for moving k8s to 100 nodes with the touch of a button pretty easily that is coming/is here. That is the power of a full stack that many people are on with. It will only get easier. Writing ad-hoc scripts and combining solutions will not.
Scaling from one node to 100s is not trivial in almost every stack. I just think that the requirement that something like that is trivial does not fit the use cases of almost everyone. There's always the "Can this be web scale?" question. We are able to add nodes on the fly but we would certainly face other non-k8s scaling issues with databases that are more complex.
And as for moving k8s to 100 nodes with the touch of a button pretty easily that is coming/is here. That is the power of a full stack that many people are on with. It will only get easier. Writing ad-hoc scripts and combining solutions will not.