“What are the odds of any letter changing pronunciation over time? Pretty damned low, right?”
Knight vs cniht
It certainly does happen but it’s infrequent. What are the chances that people are going to start pronouncing M like N anytime soon? Probably close to zero. In the middle ages, populations were more isolated than now, so changes/divergences were more likely, but it’s not as if the phonetic pronunciation of most letters was constantly in flux. It wasn’t. If you look at French/Italian/Spanish, there are certainly different phonetic pronunciations of a few letters but there are more similarities than differences. Even after they’ve been diverging for 1500 years. It’s just a ballpark estimate, but maybe 10% evolution every 300-500 years seems about right.