Besides, whether or not the home button is deactivated wont affect whether the physical mechanism can be depressed -- and that's the primary fail of my iPhone 4; the home button is "sticky" not reliably functioning the way it's supposed to (after a year), and that I have read is directly attributable to an underspeced part unable to withstand the constant use.
Also, if there's some sensor that requires the presence of a finger to activate, why have a physical button at all? Just a flat fixed plate where the home button was.
In fact that makes more sense than anything. Lose the physical button. Use the fingerprint to differentiate the space from the rest of the touch screen to avoid accidental activation.
Then again, how do people without hands or fingers use it? The current iPhone can be used with pointing devices. A fingerprint required to activate button would prevent this.
I'm not sure that anyone is saying that the fingerprint option will be the only way to unlock the phone or, even if it is the selected option, that there will not be another way in, with a reasonably secure password for example. I would guess that fingerprint authentication is attractive because it is potentially faster and simpler than entering codes or passwords, while still quite secure. Those who can't or don't want to use it but still want to lock their phones can presumably just use the regular PIN method.