Anecdotally even my old 69yo father who thinks Google Chrome is the same as Google.com has covered his webcam with tape and bought a laptop with a mic hardware switch so I really don't see this as a usability problem in 2020.

What do you mean with "flip a switch for every microphone on their devices"? Why would you want to have more than one mic and one switch per device? And turning it on in hardware is way faster than in software so a hardware switch is both faster and more secure and breaking a hardware switch seems a lot less likely than custom OEM made software failing or stop getting updated. Dust and water is no problem as lots of phones already have hardware switches, including my Android phone. I really see no problem in hardware switches at all.