Why is the argument 'nevertheless invalid'? It's a simple rule that if a word is hard to pronounce for a certain people, the pronunciation changes over time. But I won't convince you, because you don't think nfs is hard to pronounce. You won't convince me either, so let's just leave it at that.
(btw, I didn't say 'inpronounceable', I just said it was harder that 'bonnus' or 'bone-oos')