Killing the PS2 would be a horrible business decision.

1.  It still makes them good money and they've had several quarters with severe losses so any help they can get is welcome.  It's all well and good for people to sit here and say "well who cares if the PS3 loses money?  BR royalties will be massive!"  But investors will get impatient and instead of hearing about future BR royalties will just focus on massive losses in the gaming division.  By keeping the PS2 alive the gaming division has a better shot of turning a profit and pleasing investors.

2.  With the Wii being weaker spec-wise than the PS3/360 devs who don't want to make a different SKU just for the Wii can throw together a version for the PS2 quickly and Sony gets some money off of SW sales.  Some people used to argue that the PS2 was helping Wii SW support since ports could be made but at this point killing off the PS2 wouldn't nullify Wii multiplat titles.  The Wii has too big a HW advantage for companies like Activision and EA to ignore so if a Wii SKU is inevitable anyway Sony can at least try to benefit by keeping the PS2 alive and having a port on there.

3.  At the current price for PS3 if the PS2 were killed off those people who would buy one would either not buy a video game system at all or buy a Wii.  People who are waiting to buy $129 PS2s will not pay $400 for a PS3!   It is in Sony's best interest to keep the PS2 afloat till the PS3 is at a far more affordable mass market price.