I'm glad that a senior adviser at Apple agrees that it is probably not a hardware issue -- because it isn't! (by the way, the "senior advisers" are the entry-level tech support people at Apple of yesteryear)
After I updated to 8.0.2 from 8.0.1 on my iPhone 5S, I again found that I was unable to use Siri over the built-in mic. As I tried to restore this capability--and finally was successful--, I realize that I may not have listed all the steps that led to a successful outcome (I know that @ambercrystal1980 was unable to obtain the same results). I'm not sure that all of the following steps are required -- but they did lead to my regaining the ability to use Siri with the built-in microphone on my iPhone.
So if you're unable to invoke Siri successfully, try the following:
1. Attempt to invoke Siri over a bluetooth connection.
2. Disable the bluetooth connection
3. [this is the missing step] After doing so, perform a soft reset on your phone [whose display should be on] by pressing and holding the “Home” and “Sleep/Wake” buttons at the same time until you see an Apple logo appear on the screen (ignore the “Slide to power off” prompt) and then *disappear* -- in effect, your phone will reboot twice. Be sure to release these buttons as soon as the Apple logo disappears, or else you may see the picture asking you to connect your iPhone to iTunes (if this happens, hold down both buttons again until the iPhone reboots).
4. Now try to use Siri with the built-in microphone. It probably will not work at first. But keep trying. It should eventually start working -- at least it did for me.