The title says it all.
The only way this will happen is if Far Cry 3 has everything that everyone wants. If it has everybody desires for example:
Map editor (10000x better and with AIs) okay maybe not the AI part
Online Multiplayer has Predator
Endless hours of SP gameplay
Film Maker (a.k.a Theater Mode) it would make sense if this was included because Jason Brody is using a camera and also the exclusive teaser trailer.
Remember these are just examples! And what ever anyone wants (not everything there are some bad ideas out there!)
Honestly the only people that will keep playing
FC: IP and Far Cry 2 probably dont have the money to get Far Cry 3 (so that means save up you're money now or miss the action)