The 8.7 million species today, suggests lateral pressure for change (over space), is stronger, in real time, than vertical change (over time). The fossil data by being small in the space dimension, appears to make the time axis appear more dominant than it is.
A lateral pressure for change is creating diversity over space for our time. As an analogy, I take a straw and blow bubbles into a bowl of milk. The bubbles expand and spread out until they reach the ends of the bowl. After that, there is pressure to push upwards in time. Fossil data does not have as many bubbles in its bowl so it needs another explanation for vertical that exist beyond the straw.