Back in 2012/early 2013 tesla accidentally exposed a page where customers could select their data option. It was clear that if the customer declined to pay for data that certain features would not be available; web browser and streaming music were the major items. However, tesla would maintain a data connection to the car.
The fact that voice command uses data is an implementation choice made by tesla. Mobile data is not inherent to the value of the feature. As such, I would expect it to be on the list of features that continues to work if the customer declines data.
The only feature imho that could go either way is remote access via the phone app. It's extremely low bandwidth, and the marginal cost is negligible. However having data is inherent to the value of the feature. It's also likely the killer app. So if tesla wants to increase the take rate, the right choice is probably to require a data plan for this feature. If the priority is to provide a superior customer experience and highlight that the car is leading the way on tech, it should be included. In 2012 the model S was one of a small number of cars that offered remote connectivity. Today all the luxury brands do, which is also an argument that it should be included to remain competitive.
You are much more optimistic than I am, I don't expect anything that is customer facing to work without paying for data and that includes voice commands and the app. I would however like to be proven wrong, but I will set my expectations low so I can be surprised.