Perception is a factor

Often teeth get reduced as little as possible and yet a patient will perceive it as completely whacked off.  While less than 1 mm may have been reduced, the tongue will make it seem as if there is nothing left.

The end result is all that truly matters.  Enamel may have been removed, but it was replaced with porcelain, and may be in a stronger condition than before.

If there was severe crowding, severe decay or large old fillings, sometimes there is no choice but to be more aggressive.  And while a dentist could look at the current condition and make an opinion, they were not present at the initial appointment to know what the previous dentist had to work with.