> I replied to someone else in the same vein but having had a garmin watch in the home there was nothing that it would have done better if it was able to work with Apple's proprietary stuff

Maybe to you. Garmin watches cannot respond to notifications on Apple devices, for example. Detailed article about restrictions on iOS from Pebble: https://ericmigi.com/blog/apple-restricts-pebble-from-being-...

> If random devices of unknown provenance were able to freely connect with Apple devices then the security of Apple's ecosystem would take a hit. This would be bad.

Random devices are not able to connect over bluetooth to your device without your consent. Then, the bluetooth device can only get as much information as the companion app will allow it to get.

Besides, we have that on Android (and PCs) and the security of these "ecosystems" is not worse because of it.