To claim that all environmentalists are catastrophists is as absurd as claiming all religious persons believe in imminent armageddon. Whether the ecosystem is teetering on the brink of collapse, or whether it is being degraded in a gradual but relentless fashion, it surely is under assault. Many species have been lost in my lifetime and many more will be lost.
When we talk about "ecosystem collapse" there can be multiple meanings. The most extreme meaning I could ascribe to it is that our environment would no longer be able to support any of the humans who live in it. In other words, human extinction. Well, that's pretty absurd, most would agree. But if you change the definition to "no longer able to support all of the humans who live in it", then we have reached that point ready. There are populations in parts of the world who are suffering greatly from environmental degradation.