I think songs both evolved and devolved, and that is still the case, albeit at a slower pace.

Think of how many different versions there are of John Barleycorn. The older versions reflected the actual process of making beer, with the correct names of the various participants, whereas many newer versions change things around so the the beer "recipe" is made into nonsense.

There also seems to be a process of standardising choruses in songs. I don't mean between songs but within a given song. One example that comes to mind is Sandy Denny's Who Knows Where the Time Goes. The last line of each verse, as written, changes each time: "I have no thought of time", "I do not count the time", "I do not fear the time", yet many singers just use one and repeat it. Sad, because it loses a subtle but definite change in meaning as the song progresses.

I wonder too how the existence of lyrics sites plays into it. So many of those sites have the lyrics set down by a third party who is trying to get them from a recording and so many of them have nonsense words or phrases just because the scribe couldn't make out the actual words (obviously Mudcat is exempt from such a charge!). Do these errors get propagated or sung, I wonder?