Hawking is very attached to his voice: in 1988, when Speech Plus gave him the new synthesizer, the voice was different so he asked them to replace it with the original. His voice had been created in the early '80s by MIT engineer Dennis Klatt, a pioneer of text-to-speech algorithms. He invented the DECtalk, one of the first devices to translate text into speech. He initially made three voices, from recordings of his wife, daughter and himself. The female's voice was called "Beautiful Betty", the child's "Kit the Kid", and the male voice, based on his own, "Perfect Paul." "Perfect Paul" is Hawking's voice.
Something like this could let someone keep their "original" voice. Hawking may have preferred to have something that sounded like his voice before he lost it than a completely new voice.
I wonder what sort of recordings and other data you'd need to get this right assuming what TTS might look like in a few years' time
The impact on his quality of life - imagine not being able to communicate at all - would have been massive were it better.
I expect it will be a while until we can fully utilize that data, but I have to imagine that something could be done today to preserve my voice (while I am still in my prime). Effectively, this would be a sort of vocal cryogenics, betting that we can do something today that will allow us to take advantage of future technology.