You're totally right. From what I hear products like Alexa are pretty impressive, and as everything becomes more connected- hopefully with fewer problems than we're dealing with now- and more functions are offloaded to AI assistants there'll definitely less reason to have a phone on hand. I don't think Apple's oblivious to that either. Apple's positioning Siri more forwardly and the Watch has room to grow. One Alexa on your wrist (or in your ear or on your glasses or hell let's wear Star Trek communicators) is better than several scattered around your home.
But Apple has a big advantage ther with HomeKit. It is already pretty well-established, and has many advantages over other IoT implementations. Security being the biggest one.
It will be interesting to see whether Apple continues to position the Apple TV as their HomeKit Hub, or whether they actually have a more advanced Home Server product in the works. But either way, it will rely heavily on the convergence of Siri and HomeKit technologies, IMHO.