The advantage of this sort of DLC is that they DON'T have to release a new game. It's far, far more profitable for Sony to release a full games' worth of DLC rather than release a whole new GT. Now, I'm sure we'll see GT5 - i'm not saying we'd just sit downloading cars and our whole lives. But I would fully expect to see cars, tracks, parts, etc available for download approximately 3-5 months after release (right when the more casual players are ready to move on.)
PGR2 followed this plan. They released a set of downloadable tracks and cars on Xbox Live for $5. I bought it, hundreds of thousands of people bought it. Sony looks at this - the Bizarre people had to do what, code a couple more cars and a track? No fees for creating discs, no marketing campaign, no retail distribution strategy - probably took 3 weeks to make the cars and it's an instant $500,000 in the bank. Xbox Live and the new PS online system aren't for gaming - it's a new distribution channel. Now, 3 weeks later, they release another new pack for PGR2 - another 5 bucks. (Not from me this time, but from the data it looks to be selling well.) Obviously there are diminishing returns here, but this trend is a major focus for game companies.
And Jaguar - I don't mean to single you out, but Sony will definitely not be losing focus on the PS2

Their estimates indicate at least 2/3 of the PS2 sales (hardware and software) still remain. This Christmas will be a big PS2 push, as will the next one, and probably the next one. They would rather see a smooth overlap of console generations rather than the huge cycles we see now - keeps shareholders and accountants much happier. The software companies also see sales continuing on the original PS, and many are unhappy they moved their dev teams to the PS2 so quickly. Expect to see many hire new devs rather than cut into their old teams until 2006 or so.