To be honest it's really a point of how well the PS4 itself will sell. Sales have been rather strong compared to the PS3 but even going at a hearty clip it will take years to catch up the the instal base the PS3 currently holds.

If the market demands it SE will drop PS3 support but it wont be for years to come. They are going to do whatever is more profitable for them, not what would give them more breathing room to develope with.