The assumption that because the meaning of "rock" changes so does the meaning of "folk" or "RnB" is ill-founded. "Rock" (etc) are stylistic definitions (if they can be called definitions). "Folk" is not. Go and read the 1954 definition again. It depends on source, and on medium of transmission, which are not as subjective as style.

If the expressions "folklore" and "folk arts" and "folk dance" have meaning, why not "folk song" and "folk music"?

RnB seems more arguable. At its start, it was largely synonymous with "race music" which did depend on an objectively (I'll make that "almost objectively" since framers of race laws the world over have found the defintion of race a bit slippery) definable source. If that was the function of the term, then current usage is incorrect.

THe problem with "Acoustic ROck" is two-fold. THere is much current music that is getting called "folk" that in no rational sense can be called "folk" so an attractive label is needed. THe second is that simply by counting examples, one can discern groups of stylistic patterns or formats for known types of folk music. But the adoption of that format does not make music folk music.