I would like to suggest that your next device be an HVAC Thermostat control unit with ZigBee/Z-Wave hub/router integrated into this. That way, you have a dedicated control on the wall that belongs there, and we can pair up these remote devices to it. The added benefit is that the thermostat becomes the central home hub for managing the home. Remote temp sensors and motions sensors can be used to fine-tune the temperature control and HVAC cycle (like how Ecobee 3 does), contact sensors can automatically turn on and off the HVAC if configured, and if it comes with a good enough touchscreen display, it can also be used as a video/audio interface to cameras, microphones, and camera doorbells.
With most houses having the thermostat relatively central in the house, it should provide good enough connectivity coverage, also. Those houses with multiple units would have per-floor coverage. Making the thermostat/hubs able to share information between the units would be best to create a single "mesh" HVAC/ZigBee/ZWave control. That way, each thermostat would act like a hub and a router (might need to make the thermostat a device as well that can route), and all remote devices would communicate with the nearest thermostat automatically.
Idea has merit, however I have two counter points.
I've found that something that tries to do too many things, does nothing well.
And, I like having control over where in my house the hub resides.
Yeah, the key issue with putting a hub in a thermostat is, the location of some of our thermostats just really is not ideal for a home automation hub. The idea of building this into your router has merit, but I think it's still wrong. I'm replacing my routers every three years on average. That's a much faster cycle than a home automation hub needs. Especially one like Hubitat that has the chops to expand and sustain itself well beyond that mark.
I agree that complexity makes things less ideal, but in this case, my Ecobee is doing the same thing as a dumb thermostat + Hub + IoT sensors and control app. That to me isn't any different than the Hub controlling a regular thermostat. Can't do much about locating the hub/thermostat, though. But I could argue that having repeaters/routers may mitigate the location issue.
While upgradability may be an issue, but as long as you have a bunch of remote devices using a specific standard, you're not likely going to change standards without taking a big expense of swapping everything out. Therefore, the idea of upgrading hardware may not be that big of an issue. If you're talking about upgrading the hub/thermostat hardware, that may be overcome with software. Software can be upgraded OTA, as long as the hub is capable of general processing, which it should be since it's running apps locally.
My ideal home automation device would be a wall-mounted 8" tablet where my thermostat is, giving it power and replacing my need to manage multiple things: thermostat, hub, control unit (although I'd use other devices along with the display).
Unless you have a Carrier Infinity system . . .