A good ADC won't do anything about aliasing. You'll get some aliasing any time you have any input frequency components at or above the Nyquist frequency. You can't completely eliminate higher frequencies with a practical filer, because realistically it's going to have some kind of roll-off curve. If you want to preserve just the pulses, you need to sample with 1-bit resolution at a substantially higher rate than twice the bandwidth of the FM detector's front-end. It's not as straightforward to do this in the real world as you seem to be implying.