See, the point is that even your claim that new and undiscovered species are thriving is not scientific.
There is no scientific evidence in favor of this statement, therefore the whole discussion is based on conjectures and opinions.
As far as I'm concerned, knowing that the extinction of plant species goes on so fast worries me.
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.