Do we really know what they think? I suppose we know what they say. To the question of the article, let's say that one or more of them are actually fairly certain that collapse is coming within a few decades - what would be the point of telling anyone that?
But anyway, the whole idea that scientists can predict the collapse of civilization is wrong. At some level this is a question of optimism vs. pessimism. That's not a question of models, it's a question of taste or, as the kids say, "vibes".
If someone were of the honest opinion that broad scale collapse was imminent I can only expect they would also not expect fame afterwards. Anyone sharing this opinion disingenuous is more likely to be sharing it for gain before the collapse that may never come, via book sales or speaking engagements for example.