As in biology, the boundaries between dialects and languages can be difficult to ascertain and can therefore be set arbitrarily.
There are many regional dialects of German and which are considered by Germans to be very difficult to understand. We also saw the same thing with English in PBS'
The Story of English where Englishmen speaking their own dialects had to be subtitled for the audience to be able to understand them; this was done to comic effect in
Hot Fuzz when a chain of two constables were needed to interpret what a local farmer was saying (the constable who could understand the farmer could not be understood by the sergeant, so another constable had to interpret for him).
In the general region of Belgium and the Netherlands alone there are several distinct dialects of German. So when do they stop being a dialect of German and become a language in their own right? According to the encyclopedia I grew up with, Dutch became its own language about 400 years ago
when it developed its own literature. Nothing changed in Dutch at that point except that an arbitrary decision was made about its linguistical status.
So then the same thing could have happened with French, in that a change in how it was used, namely that it started being used in literature, caused an arbitrary decision to be made about its status as a language. Says nothing about how French had developed over time.
™ Version 4.2
