Theres no way Sony will instantly drop the PS2, its still the best selling games console. IIRC they said that they want to make it last up to 10 years, so should have a couple more years worth of good games yet. Of course there won't be any massivly successful titles anymore, but still some good ones. No doubt GTA: Vice City Stories will be heading to PS2 eventually, maybe even a GTA: San Andreas Stories.
You can always replay old games aswell, or play the ones you missed.