SonosNet will not be developed any further. All latest Sonos products don’t support it any more.
And frankly, any WiFi signal, be it your own, or the older SonosNet, is always subject to being hacked, by its very nature. The question is, what does it carry, and what does it have access to? These are things that will determine whether a hacker would be interested in expending the energy to break in to it.
At inception in 2005 SONOS had its own mesh, called SonosNet. Home mesh networks were unknown at that time and WiFi was much cruder than current WiFi. At this point home mesh is available and WiFi is much more capable. There is no point in SONOS expending the effort to maintain SonosNet for newly developed product.
Seems to me the business case for creating a Wi-Fi mesh to serve not only the SONOS speakers but all devices connected should be explored to help the company expand into the future.
I think that a good approach would be to develop a “certified with Sonos” list of WiFi manufacturers. This could work similar to the music service offerings. Once a music service is certified, it will be included in the SONOS offerings. For WiFi it could be a list of WiFi systems that have been shown to work with SONOS. These systems could have enhanced customer support both by the manufacturer and SONOS.