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.
Yep, that's the question.
Sadly I don't think we have an answer yet.