Actually, there is a way out of the conundrum...

The appropriate question is:

What is the rate of genetic change in plant life?

This question is still a bit broad for an answer, so one should ask:

What is the rate of genetic change in grasses or small short lived shrubs?

The answer should have the form of: Y new species per X generations and then we could have a discussion about something scientific as well as answer the question of extinct species versus new species.