skyfire, I disagree with your legacy theory. If anything it prolongs the life of the prior console. The ps one, remade console, was released in 2000 just before the ps2 and sold well into 2003 and 2004 based on it's price point and the idea that people could make ps1 games later into the cycle and have them bought by ps2 owners.
I agree backwards compatability is a selling point. I just think it's somewhere way down on the list. If backwards compatability can make up for a poor launch, why do we always hear about poor launches? I mean if the ps1 games were such a great selling point for ps2 we shouldn't here any complaints about the launch because the backwards compatability filled the gap. Just like ds. Why should we care that we only got 6 games at launch because we can play 200+ gba games? I think the reason we care is because the launches sucked and we buy next generation systems to play next generation games which these systems sorely lacked.
I really don't think that the ds would have sold much less. It was the thing to get for the holidays. Regardless of Nintendo's marketing the ds was hyped up during the Christmas season. I think it would have sold everything it had for Christmas, maybe not as much early this year if it wasn't backwards compatable.
Reasons I think the ps2 was so successful.
No direct challenger. Ps and n64 were at the end of thier life cycle and people were ready for the next generation. The dreamcast was pretty much an immediate failure as I believe it launched in 1998 and had almost all support dropped by 2000 when ps2 came out. Nobody really knew that Microsoft could be a serious contender yet and Nintendo had a ton to overcome after the weak sales of the n64.
Getting to the market early. I think that ps2 pretty much had a 10M lead by the time that Gamecube and Xbox were released. Not only that but ps2 was available in the Christmas season of 2001, which gamecube and xbox weren't because they were sold out. That allowed ps2 to carry back to back Christmas season in sales.
3rd parties love playstation. Early to market, easy capitalization of optical media. Ps2 had a weak launch, but it's competitors didn't exactly have great launches. Ps2 easily had 4 times the games out at the time xbox and gamecube launched.
DVD playback. This is when DVD got hot. Including it was geniuos. Not only do I get a nextgen system but I don't have to pay $100 + for a dvd player(at the time).
Hype. Playstation is just the best marketer of the current gen. They have become generic for video games.
Other benefits. Whatever other reasons you can think of owning the ps2, this includes backwards compatability.