It's a very interesting point, and I honestly do think that, especially as regards Sharpe vs. Aubrey, the latter is preferred as more realistic to its period. But, man, do I enjoy a good Sharpe novel.
My point is that we cannot be completely honest unless we admit that, having not lived in the time period in question, we can only write, if we wish, to our
concept of the period and its attributes and read by these same inferences. These conceptualizations change over time based on new discoveries and such, but can be filled with all sorts of assumptions that may or may not be true but have become the standard of thought. Witness the belief that upper class did not use contractions: is that true or do we simply know it was less common to use contractions in writing the dialogue for such characters as a common marker to indicate such characteristics? Or the standard vision of the dancing as being slow and sedate when research indicates it may have been more of a rollicking good time than we'd imagine, and that some of the gallops at Almack's resulted in gentlemen piled up at the end of the line (I could probably find the citations for these if I bothered, but I feel too lazy this morning to link).
Whether one absolutely
must write in the period, to the degree they can, is debatable and really up to the writer in the end. Some writers, knowing the above, may simply say "Blow it -- I like humor and plot more than pure accuracy, and I think my readers do, too. Let's have fun with this." More power to them, I say, especially if they're successful at engaging the reader in a good story, because that in the end is the whole point.
I hope I never ridicule what is wise or good. Follies and nonsense, whims and inconsistencies do divert me, I own, and I laugh at them whenever I can.