Rock and Roll is a derivative of blues.
There is nothing conservative about definitions according to the dictionary, definition can mean the degree of distinctness in outline of an object, image, or sound.
Good enjoyable music frequently falls into a definition, for example Mozart concertos are classified as Classical, Beethoven concertos are classified as Classical, Rock around the Clock is classified as Rock and Roll,Robert Johnson is defined as a Delta Blues singer.
If i want to hear Blues and see it advertised as blues,I do not want to hear a Cliff Richard impersonator, neither do I want to hear a heavy metal band or a rock band like Status Quo.
If I open a can of Tomatoes I do not want to find it contains Corned Beef
If I see punk rock advertised, I do not expect to hear a Country and Irish band or a band doing covers of The Spice Girls.