The PS2 is happy to sell even today because it was the logical choice for developers last generation, and there are many quality games for it, and still smaller developer and B-teams are pumping out games for it (althoug they are quickly shifting to the "shovelware" category)... not because it is called "Playstation", as you and many other posters seem to imply. If the Playstation 3 remains a Gamecube in spirit - inferior ports, some first-party gem here and there (btw we still haven't seen one - 80-ish Gamerankings games don't count as "gems", Gamecube was much better in this regard), modest market share - it won't sell well in its seventh year. A console only sells well in its seventh year if it sells 100 mln in its first six - you can call this law after me ;-)
The point was less to highlight why the PS2 is selling, but that it
is selling (and that, longer than any other console has) and has proven to be a very lucrative market for developers.
Each generation so far as proven to be a huge financial undertaking, not only for the platform vendors but especially publishers and developers, that have to grow accustomed to new hardware, new libraries, new approaches, make new assets etc. Big financial undertaking also mean higher risks - especially when development taps into the multi milions and extremly large teams. We've already had talks before this generation shift as to how publishers are meeting Microsofts early entry with the Xbox360 - that it's too early for the industry etc.
Begin to imagine what it'll be like next generation!
When there were reports stating that Sony wants the PS3 to have a life span of no less than 10 years - I don't think they were joking. While Microsoft may be already slowly gearing up for a new Xbox sometime before 2010, I think Sony will be still happy to sell their PS3 a lot longer if the market allows. For that to happen, they obviously have to get the price down, get more software outthere to gain those sales that are still lacking at the moment. If they can achieve that before Microsoft gears up for the next Xbox, I think the PS3 platform could prove to be a very lucrative market down the line. How many developers would even want to rush into next (next) gen development if there are still plenty of reasons to support both the xbox360 and PS3?
Most money is made towards the end of a generation, simply because the hardware is slowly tapped out, it's well known, the market is large and a lot of pre-existing assets and libraries can be re-used by developers which in turn makes development cheaper and earnings higher. Clearly, as development costs are rising and becoming more expensive by the generation, less will be willing to jump onto the next generation early, unless they must.
Not saying that Sony will achieve those numbers or do it this way, but think these are some valid points to consider. The race hasn't been concluded yet and the X360 hasn't sold beyond Xbox1 numbers yet. Sony can still pick up on sales quickly and if they do, I'm willing to bet the PS3 will stick around for a long time...