I think that is not a good comparison as it has just been in the last year or two that buying tv shows on DVD has become common place. Think about it this way, Angel Season 1 just came out last month, and I believe Season 2 is coming out in September. If they keep that pace up, Season 3 will come out in February 2004, Season 4 will come out in September of 2004 and Season 5 will come out in February of 2005. That would allow for Season 6 to come out just as Season 7 (if we make it that far, but come on, sure we will) is starting. Who knows if the will keep up that pace though.
In the future I bet you'll see tv seasons released a year and a half after they were first aired, that is they will be released halfway through the next season. That's the way it has been with shows like The Sopranos (excluding the huge break between seasons 3 &4).