Why not a manual switch? I rarely use the microphone on those devices.
Users would have to remember about turning it on/off, many would forget, leading to frustration.
Also Apple would have to place that manual switch somewhere, this would change the device's interface.
Lenovos used to have a physical wireless disable switch, and I recall it only causing a moderate amount of frustration and I doubt much effort was put into making it problem free.
The last time I've seen a hardware switch fail was 2 years ago.
Further to that, hardware failures are predictable. Lifetime of switches is well know, and they are easily replaced. Good luck fixing a software glitch by yourself as a user.
Consider that a laptop already comes with a hundred switches in the keyboard, and they can easily last a decade if you dont spill coffee on it.
Keyboard is a different kind of switch and they are major source of pain for many users. My macbook is broken because of keyboard, for example. One reason why new hardware devices like phones and tablets are coming without keyboard is this reason. Apple is not "brave enough" to replace macbook keyboard with touch panel, but they would do that if users would accept it.
Large tablets have been around for nearly a decade now, and people that write novels and word files all day still use a real keyboard. If what you are saying was true, everyone who types for a living would use iPad pro or equivalent.
Secondly, patches and reboots are a problem, not a solution. Users do not need to replace switches themselves. My father is not an IT nerd, he uses his laptop for work. After the keyboard broke, he took it to a nearby repair store and they replaced it for $20. If in a few years it breaks again, he can do it again (ofc laptop might be worth replacing for performance reasons, but that's different).
To him software crashes are far more disruptive, he cant call Acer or Apple and give them $20, $2000 or even $w0,0000 and get them to fix a sound driver or whatever.
He just has to sit there and hope that some day one of the updates, without proper patch notes, will fix the problem. Leaving feedback on the forums is about as reliable as praying for rain or doing blood sacrifice to cure the plague.