Yeah hot water can leach metals from the pipes that you really don't want to be drinking. This could either just taste bad, or add unhealthy amounts of lead and copper.

I would not use this plumber.

We are fast approaching a time when all houses have pex. At this point we've been using it in new construction for decades, and most houses with original galvanized pipes are either already replaced with pex, or getting really close.

I'll be honest, I also am not enthusiastic about drinking the volatile organics that hot water leaches out of polyethylene piping.

Give me copper or give me death.

I think you are vastly underestimating the age of the average house.

Anecdotally, sometimes your hot water heater or holding tank will rust off a piece and turn your water iron-rich.

Sometimes houses with water softeners will run non-softened water to the cold tap on the kitchen sink for cooking purposes, as some people don't want the salted water.

Correct. Water heaters fill with sediment and should be flushed annually. Of course, nobody does this.