Does Home Assistant + Mic replace the need for a device like Google Home?
It doesn’t, no, you still need the “smart” part of the smart assistant, HASS can respond to a smart assistant and perform actions but doesn’t do the voice recognition piece. If you are looking for a cheap/homebrew equivalent to Google Home, Snips might work - it has support in HASS.
I saw Paulus teasing some updated voice control for HASS coming soon, but it wont replace your Alexa/Google Home yet.
I use Homebridge (with hass plugin) + HomeKit + Siri and it works pretty well.
Can we perhaps replace the conversation microphone to launch the google home voice intent from the ui. If that could trigger com.google.android.apps.gsa.staticplugins.opa.hq.OpaHqActivity