I'd have expected there to be a mention that [abominations are] becoming slightly more common, but do we see evidence that that's not the case? In the absence of evidence either way I'd have guessed that there are more abominations forming and fewer are being killed soon after being formed, unless the free mages still employ some of the Templar protocols for controlling them that I'd been afraid freeing the mages would have ended. (Such as the mandatory education and the Harrowing.)
As a scientist, I can tell you how to definitively prove something.
You begin with a hypothesis. Your hypothesis is that abominations are becoming more common.
You design an experiment, and you start small, a binary yes/no scenario, where you compare the results obtained from your test subjects to those of a control group. Your experiment is the game, where you will observe the number of witnessed and alleged abominations. Your control group would be previous games, novels, and comics.
You collect the data. How many abominations do you see? How many people submit verifiable reports of abominations?
You analyze the data. How does the number of abominations in the test group compare to the control. You seem to have done all this already, and come to the conclusion that there are no more abominations/reports of abominations than there were in previous installments. There are actually considerably less (pmuch an entire Circle in DAO and pmuch the same in DA2), but never mind.
If the data does not support your hypothesis, your hypothesis is wrong and you need to revise it. But most people would rather believe that the evidence is wrong rather than the fact that their hypothesis is flawed. It's probably human nature.