If the science doesn't support a deadline, then you don't pull one out of thin air while passing it off as a scientific conclusion. To do otherwise is dishonest, even if you think you're doing it for noble ends.
The estimates might well be genuine at the time of publication, but our response to them will make the estimates inaccurate. Maybe all we're managing is to maintain equilibrium rather than positive progress.
I don't assume intentional dishonesty here because the risk of being completely discredited is way too high.
We're talking about the prediction "These glaciers will be gone by 2020".
And I'm not going to subject myself to a bunch of science denier sites to come back with a list of other failed predictions, but there are quite a few of them. Ones that are far more dire and breathless than a tourist trap going away.
If there's a risk of being discredited, it doesn't appear to apply to this topic. Intentional dishonesty? Nah, this is more like depraved indifference. There are no consequences for speaking out one's ass, and most attempts to enforce those consequences socially gets one branded as a "denier".
I don't think that's what people like myself are saying. We're saying that if you can't make accurate predictions (which there's ample evidence of), to declare you "know" what will happen is folly and self-aggrandizing.
"If you're in any way even slightly wrong, we're right."
Denial mixed with pride is a powerful drug and you see it in action everywhere; the amount of energy we expend on avoiding an uncomfortable truth, be it unrequited love, losing a loved one, or being found to be wrong in an argument.
It doesn't really feel like we're fighting for the survival of our humanity. Maybe just prolonging our own individual existence so we don't have to deal with change, our beloved children be damned.