The "mechanical keyboard" label is mostly arbitrary. Only somewhat reliable definition that matches with how it's widely used in context of keyboards that I can

The "mechanical keyboard" label is mostly arbitrary. Only somewhat reliable definition that matches with how it's widely used in context of keyboards that I can think of is "everything except, rubber dome, touchscreen and capacitive button keyboards (the ones without any moving parts, not the ones where keycap+spring moves one of capacitive surfaces)". Any attempts at defining it based on meaningful physical construction element or property will likely result in rubber dome keyboards classified as mechanical or some of the switches in "mechanical keyboards" as not mechanical. According to your definition about point where force decreases -> half the mx keyboard switches are not mechanical because they are not clicky and have linear force curve without buckling point, but the rubber dome keyboards are "mechanical" because they have a point where dome buckles and force decreases. If you look at the nature how the electrical contact or signal is made. Again rubber dome keyboard actually mechanically connect and disconnect the electrical contacts. But the few commercial "mechanical keyboards" using optical or magnetic switches which from the mechanical construction is almost identical to "real mechanical keyboards" would fall on the other side of split. That said there is some use in classification of switches outside the context of keyboards based on whether they have any moving mechanical parts, or whether they contacts which get connected/disconnected during use and could increase resistance over time or spark in case of higher voltages and currents have their use in certain situations.