Why dont we ditch the old radio system full of parasties and use ADS-B instead for voice communications instead?
Well, why don't we use televisions for washing dishes? Because televisions don't wash dishes.
Likewise, ADS-B doesn't carry an audio signal, so it can't be used for voice communications.
Perhaps ADS-B could be modified to carry voice signals, but this is probably not a good idea. The reason for this is that, while aviation voice communications currently use a band which is 19 MHz wide, the band that ADS-B uses is only about 1 MHz. (The Signal Identification Guide says, "There are two types of ADS-B: one [...] with about 50 kHz of bandwidth;
[and] one that transmits at 978 MHz [...] using a larger bandwidth of about 1.3 MHz".)
Transmitting voice signals digitally is a good idea, but I don't know of any reason why this would be done using ADS-B. It would make more sense to simply design a new protocol from scratch.