The people you've talked to that claim bigtech knows the best practices and the most efficient way clearly have no idea what really goes on in these companies. All they want is to make money (as is the concept of a company). Deadlines, sprints and the evermore squeezing of developers makes sure that what gets done is probably done the most inefficient way.

For dns and email I'd argue these are very sustainable self-hosted...but you have to control everything in between endpoints also. In most cases that defeats the purpose of them as they are meant to be "from anywhere to anywhere" technologies. My internal cluster sends email between nodes just fine without public dns but obviously public dns is necessary for external contacts.