That seems like a non sequitur to me. It seems like you're addressing a completely different question. If the question is "How easy was it to find good current music during the 1980s?" then yes, the 1980s might lag behind other decades due to much of the best music being non-mainstream and no ability to use the Internet to discover music.
But, to me, that isn't the question at hand. The question at hand is "Did the '80s produce less good/great music than other decades?" In evaluating that question, it's perfectly reasonable to look back and apply modern understanding. Whether or not that music was readily available to most people at the time (some of it was, some of it wasn't), it was all produced then and is readily available to us today. As far as the '80s contribution to our current pool of music, there was a tremendous one.