Modulation classification via demodulation confidence?
try decoding the signal using various modulation schemes and see which gives you the most confidence


well, simply because your "confidence" measure is basically equivalently hard to design generally enough as the other methods you mention.

Also, the search space is simply too humongous. How would you decode all possible combinations of constellations, symbol rates, with unknown phase and timing offsets, over all possible combinations with multi-carrier systems without any prior attempt to narrow things down? How low would your detection thresholds have to be so you could classify even without having done system-specific synchronization (which you can't do, without first assuming a system)?