You do generally need a tuned filter before the rectification, unless you have an extremely large signal dominating the local airwaves. Which is precisely the parent poster's point: with RF you are almost always doing something to demodulate the signal. Whether you are doing it with a sine wave or something more complicated is not that fundamentally different. (and if you're looking at a spectrum analysis, that is looking at the radio signal from the point of view of that sinusoidal modulation scheme, so you will see such signals 'above' the noise floor more readily than something using a different modulation).
No? Old AM radio required only rectification. You can receive it with accidental diodes.
You do generally need a tuned filter before the rectification, unless you have an extremely large signal dominating the local airwaves. Which is precisely the parent poster's point: with RF you are almost always doing something to demodulate the signal. Whether you are doing it with a sine wave or something more complicated is not that fundamentally different. (and if you're looking at a spectrum analysis, that is looking at the radio signal from the point of view of that sinusoidal modulation scheme, so you will see such signals 'above' the noise floor more readily than something using a different modulation).