I think it’s a misrepresentation that environmentalists think humans don’t “deserve” the planet due to our poor treatment of it. It is true that many of those concerned with the environment do believe that a concerted effort of humans in general would help the problem. You say that the position of environmentalists is that humans are inherently poor at dealing with the environment, that we “reek of pessimism”.
Your way of dealing with this view is to say that the environmentalists are contradicting themselves, like they think we ought to do something, but know that we’re incapable of it. This is a misrepresentation.

Your take on this:
1. Environmentalists think that humans are naturally inclined to damage their environment (to the point of raising CO2 levels, or whatever other scientific evidence an environmentalist would cite when making this claim)
2. Environmentalists want humans to stop damaging their environment and believe they have the capacity to do so (or else they wouldn’t campaign for anything)
3. This is absurd because 1 and 2 are mutually exclusive
4. This is the Mother (Earth) of all contradictions
5. Therefore, environmentalists have a flawed starting point

Point 1 is of dubious truth. We all produce waste to a certain extent and an environmentalist would acknowledge this. That we all destroy the planet beyond redemption against our will (because we’re naturally inclined to do so) is not the position of all environmentalists, though, not by a long shot. Aside from perhaps the most basic hereditary instincts required to survive, we don’t have any sort of a priori moral compass, in dealing with the environment or with any other ethical issue. Perhaps the only moral “truth” we can come to agreement on is that, in general, people like to survive, and that, in general, all species strive to survive.

Point 2, which you suggest environmentalists uphold, is almost certainly true, even if you asked an environmentalist.

I fail to see, though, how your subsequent points (3, 4, 5) logically follow. Even if we accept 1 as true, I don’t believe this is sufficient cause to discredit the argument of environmentalism. You cite it as a contradiction.
Consider the case where all environmentalists acknowledge the truth of 1: we’re all bad for the environment, naturally. The environmentalist, however, would submit that he or she is not bad for the environment, as he or she makes conscious efforts that align with whatever principles about the Earth he or she has decided are good. So, he or she acknowledges that, though we may be inherently bad for the environment, we can still come to our senses and decide to behave in an Earth-friendly way (the environmentalists themselves must necessarily have realized this at some point in their life, given that they were inherently bad for the planet). So, there is the possibility that even the worst Earth-destroyer could come around and side with the environmentalist. Therefore, the environmentalist’s efforts are worthwhile for his or her cause.

For ease, I have reconstructed a more accurate perspective of an environmentalist:

1. Humans, as we exist now, are damaging the environment which may jeopardize our long-term survival
2. Not surviving is undesirable for me and for most humans
3. There exist ways in which humans can live that cause far less damage to the environment, while still upholding certain desirable characteristics of society
4. Humans can be persuaded to change their damaging ways if they learn about the damages
5. I, the environmentalist, have become aware of these changes and believe I know what the alternatives are
6. Therefore I ought to spread word of environmentalism, in an effort to prolong our survival contra these damages

Carl Sagan was worried about the greenhouse effect, and he used Venus as an example–whose atmosphere is over 95% CO2 and ours is…well– read above. He also believed in the scientific method, and so did the brilliant Richard Feynman; they would be disgusted with the way we have exploited this issue from one side and fed it to the general population as creed.

It is despicable.