The last time I was involved in this kind of thing (long enough but not too long ago), we ran Postgres and other databases strictly in K8s per a mandate by our CTO. Using reserved instances are great for avoiding a lot of cost.

I think for that particular firm, "K8s tax" measured in fiat currency is negligible and anything on the human side their people upon hearing "K8s tax" would respond with some ridicule along the lines of "someone doesn't know how to computer".

To be fair, most of the commenters on HackerNews should use something like Heroku.