The outset is that the home button works and performs several important functions. Naturally people want to know what the advantages of a virtual button are and whether they can outweigh the convenience of the physical one.
The home button works even when the screen is turned off, it responds to both press and hold and double/triple clicks. A virtual button would have to provide the same functions. It would have to respond to touches to wake up (to mimic the wake-up function of the home button) and still be easy to use with all the accessibility functions as well. A particular disadvantage, as was pointed out here, is that it may be more difficult to avoid tapping the home button in games and other apps. We also lose any haptic feedback, which can be a good contrast to an otherwise all-touch experience.
I am confident that Apple considered getting rid of the home button multiple times and decided against it. That a physical button is prone to break is an argument, but not a compelling one if contrasted with the overall changes. Don't judge people by defending the status quo, but convince them with arguments that a virtual button is a better experience. I for one thing am not convinced of that yet.