Useless is a strong word in this context. Which parts or features of k8s are useless complexities? It's not like some random junior dev pulled a bunch of features out of a hat and implemented them. There was a ridiculous amount of thought put into its features and I can't think of a single complexity that is useless or even one that is useful but could be done in a more elegant /less complex way.
Yes, it is complex, but there are lots of use cases where it is the most elegant and least complex solution. Yes, it definitely does not make sense to use it for a lamp stack deployed to one server, but there are use cases where it's a huge improvement (e.g. Spark on Hadoop is extremely complex and clunky when compared to spark on kubernetes).
Like you said, if you're good with a LAMP stack and one server, then K8S is probably useless complexity.
The issue is that most people nowadays have never worked with a bare metal Lamp server, so they grossly underestimate how large their company can grow before needing any distributed HA solution. I'd wager 90% of startups go bankrupt or are acquired before outgrowing a single Lamp server.
Which items fall in this category?