I'm not going to pick apart every lyric- there's more than enough above this w/u to cover that- but from where I stand, it seems to me that McLean was writing about momentary "deaths" in music. Such as Buddy Holly dying, John Lennon splitting from the Beatles, Elvis' career on the decline, The Rolling Stones selling out, Janis Joplin overdosing, etcetera. All of these people and music "greats" came to, seemingly, untimely ends. Each time another musical pioneer died, it seemed like the music for that day and age died, too. In a social, political, musical, artistic, philosophical, cultural and sensational sense, the music industry and society as a whole lost a bit of itself when these people disappeared from the scene.
McLean, to me, is describing pivotal points in recent history where music seemed to die, but only for a moment. Like chapters come to an end, you turn the page and start a new chapter. The previous chapter is closed, behind you- dead and in the past. There will be, I'm sure, other days when the music seems to die. Like Selena getting killed, Randy Rhodes dying, Freddie Mercury passing- when those people were lost to us forever, their music came to an effective end.
We will never have another Elvis, another Buddy Holly, another Jimi Hendrix or another Janis Joplin. When they died, their musical talent went with them and that talent can never be reproduced. Music will never die. There will always be visionaries ahead of us, but when the "founding fathers" of rock and roll pass on, it will seem like a hiccup in time where things seem to stand still and we will mourn the loss- twice over.
The song could have just as easily been about the great visual artists like Van Gogh, Picasso, da Vinci and so on, but McLean probably wanted to write something a little more current. Something that would have a bigger impact on the people who were listening to the song at the time.
At least, that's my take on it. I might be entirely wrong.