Ok but the argument is that it might not "convey the intended meaning to the listener" because 'in mass' can mean multiple things in English, whereas in French 'en masse' specifically means 'as a group'. It's not a linguistic purity thing it's literally just to prevent misinterpretation.

edit: You can't say "English is an evolving language" and then ignore the evolution in the phrase being carried over in the first place. If there was no reason for it then we'd still just be using the English term or, "banging rocks together and grunting" as you put it.