Hi. I came to realize that in japanese ん sometimes becomes m in pronounciation. If ん comes before b or p in a word it sometimes becomes m. For examples

新聞 : Read as shimbun

かんぱい : Read as kampai

コンビニ: Sometimes read as kombini

日本橋 : Even metro official writing with romaji was with m, nihombashi

But another thing is this merger is not unique for japanese. In my native Turkish this happens sometimes too. Most common would be people saying Istambul with m instead of an Istanbul.

Also in Arabic. In Ilm-i Tacweed. There is rule called Iqlab that morphs all these n into m. 

Maybe in English too. We don't say inpossible its impossible. Maybe English already solved that problem. (Inbred?? i cant imagine it said with m. Why??)

Another thing i realized that in Japanese and Arabic ん is more nasal than Turkish. And English has the weakest nasalisation.

My point is. Did this start happening in modern times because of strong nasalisation. Or Old Japanese had a m letter as a CVC consonant but it became obsolote. Or in older times they had this merger and it survived with oral traditions.

I know this is a really different question. Phonology is a whole different aspect of language. All answers are appreciated.

ありがとうございました