I don’t know, but I been told, the “mp” letter combination in Greek is similar to our “th” or “ch” in that the letters don’t represent the sound except by convention. In the case of Greek “mp” makes the sound of “b” in English. It could be a degradation, but I wouldn’t assume that. I know we have a lot of other borrows from Greek that we got wrong.
But what do they know? They also pronounce the word “Zeus” to sound like “Zeffs” Morons!
It’s not just a random assumption; cases like that don’t tend to happen when you first make an alphabet, might be noticed when words come into other languages like Latin. At any rate, the ancient Greek phonology that is currently understood allows an “m” before a “p”, with no mention of the latter changing to a “b” sound.
Look through that, by the way, and you will notice that changes to consonants in modern Greek are the rule rather than exception - beta is now pronounced “v”, gamma and delta are likewise changed, and so on. It’s a bad guide to how ancient Greek was spoken.