The problem is that actual change is hard. And hard becomes impossible in the presence of deniers. Different people want different approaches, and each of them has different costs for different sectors.
We can't even tell just how bad the arguments would be, because the presence of deniers shortcuts it. Nobody ever gets to make difficult compromises because "doing nothing" always wins out.
It's not just a matter of "it would all be over if we just did it my way". It probably would be... but that's not how things are done in a democracy. You have to take all the various groups into account. And when one large group is "absolutely nobody does anything", they get what they want because none of the actual discussions can be taken seriously.
This is all complicated by the fact that the worst deniers are in the US, and that they have outsized power here. Europe isn't especially thrilled about the idea of doing anything that costs anything -- whether it's money, their other environmental goals, or otherwise -- that the Americans will benefit from without paying for.