The m of the Latin accusative in a word like bonum reduced to a simple nasal sound, thus nasaling the vowel u. The nasalization gradually dropped away, leaving a u which changed to o which changed to schwa. Then the schwa dropped off leaving an n. Then the n reduced to a nasal sound, nasalizing the vowel, leaving us with the current situation: bon. [I haven’t checked the actual situation in French, but the course of change is something like that – this is a schematicized rendering of that historical course]
Most people don’t like that. They esp don’t like people who point out that such changes appear not to have disturbed the language overmuch, i.e. French is still an expressive language, n’est-ce pas? That really frosts people who imagine that any change must be not only a reduction in sounds but also a reduction in communicativity or beauty or tradition or loyalty or national pride or something, just something bad b/c it’s CHANGE! And change is bad.
So we all must pause here and say, “Say what?” Yes, from certain ideological perspectives, change is bad. In language we achieve this by positing a Golden Age at which the language was perfection itself: the Siglo de Oro, Shakespear’s English, Pushkin’s Russian, Dante’s Italian….. every country, nation, region has its period of perfection. And linguists who show that the great exemplars of the national tongue violated many of the shibboleth-like rules the conservationists believe are enshrined somewhere, and show also that the language contines to serve well as a medium of not only communication but of belles-lettres as well, become the object of hatred and scorn on the part of those who “plant their flags” in the service of TRADITION and STANDARDS.
Thus the origin of the conservationists’ hatred as described by H. A. Gleason in Linguistics and English Grammar.