I would. That new powerful machine is only going to have a potential 10-20 million customers as opposed to 75+ million for the current gen systems. If anything I'd develop a new game for PS3/Xbox and just gussy it up to 1080P and 3D or whatever for the next gen systems. Usually we have at least two years of games still being released for a generation after the next gen starts.
Your "whatever" shows me that you have little understanding of programming at all and equally little understanding of the differences between the machines.
Especially the case with Playstation 3 and 4 is different as both machines run on totally different architectures meaning that basically you need to program them differently and it's not a little tweak like you assume like adding a high resolution pack.
Yes.. initially the customer base for next gen machines will naturally be smaller and there will still be games for the old console for years to come but i guess given the need for efficiency, cost saving and future orientation most A titles will only be available for the new consoles.
What would be the marketing slogan for games on old consoles anyway? "Hey.. we've managed to make it run on the old machine! It may not look as pretty but there you go!" A titles will be marketed as the coolest new thing with the shiniest graphics and you just can't do that with old hardware.
So no.. any CEO who knows his job will not approve big games for old hardware if he can help it. The customer bases will automatically increase while support for the old generation will slowly fade out and if he plans for the game to stick around for a while via DLCs or a long term plan it would even be foolish to start on outdated hardware.