Yea that could work. I think this is cheaper for them surprisingly enough. Switches are expensive.
Nothing to break, zero attack surface, what's not to like?
But if you're so against moving parts, a hall sensor and a transistor would achieve the same result.
At least with the switch if the mic doesn't work you just check for magnets around it and that's it.
The chip provides a hardware disconnect for the microphone, just like your idea. Except it's baked into the silicon instead of an additional large part in the body of the computer.
Regardless, if your attack model is "T2 chip is compromised", all bets are off. Worrying about the microphone would be the absolute least of your concerns. All your local and iCloud data can now be decrypted, your 2FA is compromised, so on and so forth.
The T2 chip and Secure Enclave provide the strongest security guarantees in today's computers and mobile devices respectively. I'm not worried about a compromise of the T2 chip, to be honest.
If someone can physically modify your laptop, you've already been hacked, T2 chip or not.
if your attack model is "T2 chip is compromised"
I think the more realistic attack model is "Apple enables the undocumented 'turn on mic while closed' feature that it was required to add through a national security letter that they can't talk about."
And then that secret hack is either discovered (or stolen) by other government.