[Sabertooth]

If .01% of the observed species disappear each century, this is an estimate of the disappearance rate of all species.

Based on the assumption that unobserved species disappear at the same rate, for which we have zero evidence. That strikes me as a shot in the dark, whereas an estimate ought to be an educated guess.

[Doctor Stochastic]

There is no assumption that unobserved species disappear at the same rate. There is the assumption that the rates of unobserved species are not greately different from those of observed species. As new observations are made, adjustments can be made. None of the adjustments so far have shown anything drastic.

In real life, I'm sure you continually extrapolate to things you haven't observed.