I wondered this last time I played through. Especially considering there weren't any negative consequences to letting the Shinra try their plans out. Obviously you'd miss out on the Master Materia and Bahamut Zero, but they're not absolutely necessary to finish the game. I was tempted to let them just get away with it, though as it'd have no bearing whatsoever on the story I didn't bother.

Naturally the public executions demonstrate that the Shinra aren't exactly whiter than white and they probably are motivated out of a desire to save themselves, though they are actually doing something at least.