I'd argue the T2 chip is actually more secure than your method. Magnets can be removed. The T2 chip cannot. Not without rendering the computer unusable and the data irrecoverable.

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.