I can only speak from my experience in the UK, but history books appear to be very popular here. Something I'm happy about because I'm a history student and a large portion of my personal library is dedicated to history.
I wouldn't say historians are a dying breed. Personally, I hope to be a full time historian and I've already started the groundwork on my own project. There are also lots of young, popular historians who are best sellers, such as Dan Jones or Helen Carr to name a few. However, again they're both British. Is history really that unpopular in the US?
Regarding David McCullough, unfortunately his works weren't as popular in the UK, but I do own and have read one of his books. Last year I picked up 1776 from the Folio Society. Absolutely incredible account of a year that changed the course of the world. After reading it, I understood why he was a Pulitzer Prize winner. A sad loss for the history world, but he won't be forgotten thanks to his incredible body of work.