How to write a historical bestseller?

People say that a key feature of a bestseller is that it must be written in short sentences in modern colloquial English. (Unless you are writing in French or something, I suppose.) Of course, you want your readers to be able to read it without having to open a dictionary every few minutes. However, in the case of historical novels, can they not sound too modern? I raised this in an article I wrote some time ago about the language in Highwaypersons and previously I asked about the kind of words I am allowed to use, but I feel I need to think again, if I ever hope to write a bestseller.

A historical novelist writing an early bestseller
What about a modern bestseller?

I am also concerned that, even choosing novels set in the present, some readers might like to get away from modern colloquial English. They might want to feel authors are preserving at least some of the language. Not pickling in aspic, but not so modernising as to lose all sense of our linguistic heritage. (There’s a couple of posh words, for a start!)

A quill pen: the tool of the bestseller writer of the past
How conservative am I?

I want to see progress in many aspects of life. Innovation is good, where it actually makes life better for people. Reform is overdue in many institutions. But I do not want to lose touch with our past, whether in architecture, traditions or our language.

Have I read a modern bestseller?

People used to tell me to read a lot, because that would improve my writing, and perhaps even my speech. Is that advice out-dated now? Fortunately, I have read the occasional modern bestseller that was well-written as well as having a good story, so all is not lost. Yet!

I would love to hear what you think.