It's not for everyone and it has significant maintenance overhead if you want to keep it up to date _and_ can't re-create the cluster with a new version every time. This is something most people at Google are completely insulated from in the case of Borg, because SRE's make infrastructure "just work". I wish there was something drastically simpler. I don't need three dozen persistent volume providers, or the ability to e.g. replace my network plugin or DNS provider, or load balancer. I want a sane set of defaults built-in. I want easy access to persistent data (currently a bit of a nightmare to set up in your own cluster). I want a configuration setup that can take command line params without futzing with templating and the like. As horrible and inconsistent as Borg's BCL is, it's, IMO, an improvement over what K8S uses.
Most importantly: I want a lot fewer moving parts than it currently has. Being "extensible" is a noble goal, but at some point cognitive overhead begins to dominate. Learn to say "no" to good ideas.
Unfortunately there's a lot of K8S configs and specific software already written, so people are unlikely to switch to something more manageable. Fortunately if complexity continues to proliferate, it may collapse under its own weight, leaving no option but to move somewhere else.