Kubernetes/Docker are still fundamentally "CaaS" (container-as-a-service). Definitely an improvement from the mostly IaaS period before, but still too low level for anyone to run services reliably on with low effort. From someone who runs 400+ services on kubernetes – I'd like to make this clear: Kubernetes is not a "platform" for services, it's just the low-level container infrastructure. You have to build the platform yourself, which is not easy.
I don't think the replacement for kubernetes will be something equal or even-more-lower level (more barebones, like nomad). It would be something higher level, enabling more features, not just equal.
What next could be a proper, self-hostable PaaS. There are a few out there, but most are either closed wall (fly.io, heroku, app engine, beanstalk) or self-hostable but complicated or not easily scalable (cloudfoundry, etc).
In a way, Kubernetes also did the same thing that most of it's predecessors did. But the main difference was – it offered a common low-level abstraction of APIs and operators which allowed a lot of solutions to be built on top. It was not just a CaaS, it was also the "standard model" to run things underneath. The unit of the model was always a "workload" or container.
Similarly, the next PaaS could also do the same thing as solutions today – but if it becomes the new "standard model", where the core unit of the model is an "application" (not just a container), it would be amazing. Deploy applications with hundreds of standard, open ended plugins like distributed tracing, etc. Open ended heroku.