How many L2 speakers of English do you know who never, ever pick an unnatural choice from among the myriad of different ways in which the future can be expressed in English?
You are committing the nirvana fallacy. How many native speakers of English never make mistakes or never "pick an unnatural choice"?
For example, I know a woman who immigrated to the US as an adult and is fully bilingual. As an objective measure, I think she had the perfect score on the verbal section of the LSAT. She speaks better English than most "natives". She is not unusual.
The fact that it was told to me by a very intelligent French linguist about a friend of his whose L2-French is flawless except for occasional errors in that domain
Tell your French linguist to go into countryside and listen to the French of the uneducated native speakers. Do they make mistakes?
How many native speakers of English never make mistakes or never "pick an unnatural choice"?
I'm not talking about performance errors in general. I'm talking about the fact that it is extremely hard to acquire native-like competence wrt the semantics and pragmatics of the ways in which English allows one to express something about the future.
She speaks better English than most "natives".
Your utterance of this sentence severely damages your credibility with respect to any linguistic issue. The proper way to say this is: she speaks ...
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.