Well.. er.. Mr. Moreschi would not blend well with a well tempered instrument either.. but anyway, that's a good sample (or the only sample) that we can hear today of those famous singers. There are many stories of knowledgeable people who have had the impression of their lives when hearing the castrati in person, and this could be explained by the reasons Alice posted above.
We in our times have been born with a loudspeaker at our side, (and our singers born with a mike in their hands) and listen to artificially produced music much more than live performances, I speak in general. May be for this reason we can't appreciate the theatrical aspects of the vocal performance, the mood, the attitude in front of the audience, the warmth of the human presence. And consequently we are more strict in our criticism of singers. Singers are more careful too, and nowadays you will not find a singer who repeats less than 10 times each phrase to record a CD, the engineers cutting and pasting music to obtain perfection and loosing the necessary freshness.
The movie Farinelli is neither a good example, because despite its high quality as a film, the voice has been electronically manufactured with male and female voices as raw material, an amazing as well as artificial achievement.
Un abrazo - Andrés