You’re subtly altering the meaning of “in general public use”. As used in the court’s opinion, they’re drawing a distinction between whether the average person could perform this activity against another person. If the average person carried around thermal imaging tech that would have been capable of performing the same actions as the police, it would have not constituted an unlawful search. Extrapolating that to say that if the average person chooses to use always-on microphones to record themselves inside their homes, the police can record people inside their homes, is not an equivalent argument. It is totally viable to say that because so many people have the ability to record conversations in public places, the police can record conversations in public places. But that’s not the same thing at all.