Because every.single.one of these "integrated environments" I've ever come across was an objective mess, poorly documented and littered with tech-debt.
It was clear the "20-year SA" had forced 20-year old administration abstractions and ideas on top of modern infrastructure and application concerns. It was cheaper/better/easier to throw it out and rebuild on something like k8s than to make any attempting at "scaling" the existing solution.
You're simply trading the "in-fashion" disease for the "I'm a 20-year Linuxbeard I know best and no one tells me different" disease.
For your information I find the new breed and k8s FUD to be more of a reaction rather than an educated, seamless and practical systems approach.
Why is it better to spend money to rebuild a fraction of K8S with a patchwork of infrastructure put together by a single person?
K8S can replace quite a lot of sysadmin responsibilities and is currently doing so at thousands of companies.
If you have ever had to talk down execs bitten by the k8s hype in a scientific compute or HPC environment you would know that the marketing and sales talk is both prevalent and damaging.
It's quite simple for a 20 year SA to stand up a highly integrated environment with modular monitoring, directory service, virtualization and hybrid cloud options for all services in a week. Why don't you hire one of these for the job instead of recipe/containering yourself into 'doesn't work, I dunno' posts.