Best answer by John B
It isn't a case of Sonos 'holding on' to using the app. I'm sure that they would like to make this possible. Voice control is a work-in-progress on Sonos and this one clearly hasn't yet come to the top of the priority list.
With Alexa it is now possible to group certain speakers at the point of starting to play music, using Alexa groups. This wasn't available at launch.