MP3s are a lossy filetype, so they eventually lose bitrate and kbps over time
wait what. they only lose quality if you convert them from lossless to lossy and not vice versa. mp3 is the super god of audio compression. but it doesnt lose data magically over time...
flac -> mp3. you lost data. but its most of the time useless data
mp3 -> flac. no data gain. same quality as mp3
mp3 -> flac -> mp3. its the same thing through out. i dont think the mp3 compression algorithm forgets up so bad it compresses the previous mp3 file into an even stufftier quality