We have been using those SDKs since they became available.
Our particular app (Amazing Slow Downer) lets the user play music more slowly, since a couple of years ago also songs from Spotify. The "ios-streaming-sdk" is a beautifully designed framework that lets an audio developer do practically everything needed.
In our case it makes it possible to access the raw audio samples and this way process the sound and then playback the processed audio using our regular audio playback engine. With your new SDK this is not possible. It might be that the "ios-streaming-sdk" is more difficult to use for an average developer but that also makes it more powerful.
We have many users that uses Spotify. Many users switch from other streaming services such as "Apple Music", Tidal, Amazon Music etc to Spotify just to be able to slow down streaming music in real time.
We believe our app gives Spotify many many new subscribers, so many that it would actually pay off for Spotify to maintain the "ios-streaming-sdk".
We only use "playlist" functions and after fixing a couple of bugs in the "metadata framework" available on github, everything works 100%. Although there are certainly some bugs in the “ios-streaming-sdk”, they are very few and don’t affect our app.
We also have an Android version of our app that uses your Android streaming SDK and it also works very good.
We believe Spotify will loose thousands of users, so many that the minimal cost Spotify need to spend on maintenance will pay off manifold.
Since our app also supports playback and slowing down of streaming Apple Music, many of our users will probably switch from subscribing to Spotify to subscribing to Apple Music.
"After September 1st, important functionality in the streaming SDKs will stop working."
Exactly what does that mean? What part of the SDK will continue to work?
Please continue supporting iOS/Android streaming SDKs!