Technically you can always bring them back to life although you have to start at Gen 1 again.It really depends on your point of view.Some see each generation as a totally different entity.Each time you start it up it's a new creature and not the same as the last one you raised.
I see Tamagotchi as a single entity in a constant state of evolution.Each generation is the device's way of continuing on and staying alive,hence each of my tamagotchis has a single name ( the white one is always Vincent for example and I keep the name no matter how many gens I get because I see each generation as the same creature in a different form rather than different creatures appearing on one egg. ) I feel that the Tamagotchi itself is not just the picture on the screen but the entire egg.Since the Tamagotchi could not function without the egg or the electronics contained within I see the entire machine as the creature and the image onscreen is just a means to communicate with the device.
Of course as I said a lot depends on your point of view.I think all opinions and theories regarding Tamagotchi's existance are valid,this is just what I feel.