You're papering over the problems with relativism.
The keyboard is rapid to use and flexible. Although it has the possibility of physical health problems with RSI, with light pen interaction you have the certainty of pain and fatigue. Sign speakers don't need sign-to-speech - there are keyboards already. There's been multiple attempts at text to speech as well, and the result is more fiddly, less flexible and less rapid than you can get with a keyboard.
There could be an argument that keyboards are complicated and have a learning curve. But computers are the super-tool of our age. Why would you not learn use of a tool that gives you combines great power with flexibility.
I think we still have discussions about alternative user interfaces that hark back to the way humans interact with each other because most of the population are not yet expert keyboard users. This will change. Once the developed world is flush with expert keyboard users, user interfaces will go back to putting greater emphasis on them.
