Perhaps you will bless us with some references for this assertion.
Considering that all trilobites are extinct, it would be rather interesting to find someone who expects their diversity to increase without limit.
Within recorded history, the diversity of large animals has decreased. What makes you think diversity should increase continuously?
In rare moments of candor, even the evolutionists themselves admit this obvious fact:
Darwin’s prediction of rampant, albeit gradual, change affecting all lineages through time is refuted. The record is there, and the record speaks for tremendous anatomical conservatism. Change in the manner Darwin expected is just not found in the fossil record.
Eldredge, N. and Tattersall, I. (1982)
The Myths of Human Evolution
Columbia University Press, p. 48
You continue to embarass yourself with your failure to grasp the significance of the obvious
"Within recorded history, the diversity of large animals has decreased. What makes you think diversity should increase continuously?"
The foundational thesis of darwinism demands it.