Aliasing is the result of repeating patterns of roughly the same frequency interfering with each other in an undesirable manner. In the case of photography, the higher frequencies of the image projected by the lens onto the sensor creates and interference pattern (moiré in this case) with the pixel grid. This interference only occurs when those frequencies are roughly the same, or when the sampling frequency of the sensor matches the wavelet frequency of the image. That is the Nyquist limit. Note...that is an analog issue...moiré occurs because of interference that occurs real-time in the real-world before the image is actually exposed.

Once the image is exposed, that interference pattern is effectively "baked in". You can use software to some degree to clean moiré patterns up in post, but it is minimally effective when compared to a physical low pass (AA) filter in front of the sensor. The loss in detail due to moiré can also be greater than that lost to an AA filter, as moiré is effectively nonsense data, where slightly blurred detail could still be useful.

An AA filter is just designed to blur those frequencies at Nyquist so they do not create any interference patterns. The reason we still need AA filters is because image sensors and lenses are still capable of resolving down to the same frequency. When sensors improve to the point where the sampling frequency of the sensor itself is consistently higher than even the best lenses at their optimal aperture, then the need for an AA filter would diminish. The lens itself would effectively handle the necessary blurring for us, and interference patterns would never emerge in the first place.