The app doesn't need the cloud for bluetooth usage or basic wifi usage (though "out of your house" usage likely uses their relay service). You would no longer get "recipe of the week" stuff, but that is a small loss.

The real danger is that eventually the app goes away due to not being maintained. Hopefully they release the API spec before then, but my specific device has already been reverse engineered at least.

It should be legally mandated to release either the firmware source, or at least the API and applicable keys, before discontinuing support for a device.

I'm reminded of the Wemo netcams my parents owned, which updated to remove a previous ability connect directly to the video stream in favor of pay-only cloud software, and the proceeded to cease support for that, leaving my parents with two completely useless bricks.

https://www.belkin.com/us/support-article?articleNum=316642

The app will eventually stop working because of incompatible updates to the phone OS.

Yeah, that is the danger I tried to allude to in my second paragraph.

I hope we do see phone/OS/store ecosystems that focus on longterm stability of features and UI. Keep devices repairable by end users, supply parts for the whole of the phone's lifespan.

Fair phone is trying to do this for phone hardware, and Framework is doing similar for laptops. We still need OS and store ecosystems.

Yup, "smart" devices will require an accompanying phone of a similar age with the app already installed in case it was removed from the App/Play store -- if you want to keep using it much after 5 years.