> But I was not interested in Medieval Latin in isolation, but in how a
> gap appeared between Medieval Latin and Old French.
Approximately 800 years of divergence between the spoken and written forms
of the language. What you call "medieval Latin" was a second language for
all speakers, an artificial, learnt language. No one spoke it as a mother
tongue. There is no sudden break at all.
The earliest signs of separation have been traced back to just after the
Punic wars in Rome, about 200 BC. Plautus, writing at that time, appears
to use a language very close to graffiti and other evidence from the period,
and what we reconstruct for the spoken language, whereas Terence, writing
shortly later, shows a more artificial and contrived use of the written
language, in line with what would become Classical and then medieval
practice.
It was the enormous status given to classical Latin that allowed such an
artificial language to survive (and still to survive! Harry Potter is in my
local bookshop in Latin). We see a similar situation in Greece, with the
status of the Katherevousa allowing survival of a form of Greek very
different from the spoken language. It was only 20 years ago that
Katherevousa was finally dumped, although at least one newspaper is still
published in it.
So there is nothing to explain in the development of the Romance languages.
There is no discontinuity, no sudden jump, and no "gap".
Peter