I'd be interested in seeing if products can become HomeKit only. Unfortunately you still sort of need your own app for setup, etc. It's a decently sized barrier for manufacturers, and their app ends up shit anyway.
Most the HomeKit products I've tried don't need the native apps, just Homekit native on the phone.
Vocolinc and Wemo both are like this. In fact, the latest Vocolinc app has a "Homekit" mode, where it's basically just passthrough. Used for updating firmware only, pretty much.
I recently added a new Wemo smart-plug and used Homekit, not the app. The plug works fine in Homekit, doesn't even appear in the Wemo app. The only downside is no way to update firmware.
Biggest downside to Homekit only, is that you lose specific functionality a manufacturer might have. That said, I find Homekit to be better than the vendor stuff anyway, so stick with it.
You can actually set up some devices without downloading a manufacturer app. I did this for an iDevices thermostat I bought recently, although I downloaded their app later because the home app has no idea how to do "keep the temperature between this range during these hours". (The app is not half bad, FWIW.)
