I believe that the main reason for not being able to disable 'Spotify Connect' is that the functionality is being used to prevent people from sharing Spotify accounts. This became particularly clear in the last version of Spotify when they made it impossible to set the Spotify client in 'Offline' mode if the phone was already connected using 'Spotify Connect' (the 'Offline' setting is being grey out). Now you have to set the phone in 'Flight mode' and then start up the Spotify client and set the Spotify client to 'Offline' mode (and then disable 'Flight mode' if you want to use Internet while listening in 'Offline' mode). So my guess is that they will never make it possible to disable 'Spotify connect'.