Sure, and I do believe it is significantly worse than the IPCC is "estimating". There are many reasons why the IPCC is very optimistic.

But the result is the same: the IPCC says that we are pretty much screwed, and climate scientists say that this is an optimistic view. How does that hurt the credibility of the IPCC? The IPCC is still saying what the scientists want: we have to change, and quick.

It could lose its credibility if it said "climate change is not that big of a deal", but that's not - and by a long shot - what the IPCC is saying.