Actually I don't think you're right. I don't think there's much consensus on the issue within the community, so there's not much of a conclusion to draw:
Last year's survey answer to "which disaster do you think is most likely to wipe out greater than 90% of humanity before the year 2100?" was as follows:
Pandemic (bioengineered): 272, 23% Environmental collapse: 171, 14.5% Unfriendly AI: 160, 13.5% Nuclear war: 155, 13.1% Economic/Political collapse: 137, 11.6% Pandemic (natural): 99, 8.4% Nanotech: 49, 4.1% Asteroid: 43, 3.6%
I deliberately didn't say that the majority of LessWrongers would give that answer. Partly because Lesswrong is only about 1/3 computer scientists/programmers. Also 14.5% is very high compared to most communities.
I didn't explicitly state an argument but if I were to it would be that communities with an interest in topic X are the most likely to think that topic X is the most important thing ever. So it isn't necessary for most computer scientists to think that unfriendly AI is the biggest problem for my argument to work, just that computer scientists are the most likely to think that it is the biggest problem.
If I were to ask the question "What threat poses the greatest risk to society/humanity?" to several communities I would expect to get some answers that follow a predictable trend:
If I asked the question on an HBD blog I'd probably get one of the answers demographic disaster/dysgenics/immigration.
If I asked the question to a bunch of environmentalists they'd probably say global warming or pollution.
If I asked the question on a leftist blog I might get the answer: growing inequality/exploitation of workers.
If I asked the question to Catholic bishops they might say abortion/sexual immorality.
And if I were to ask the question on LessWrong (which is heavily populated by Computer scientists and programmers) many would respond with unfriendly AI.
One of these groups might be right, I don't know. However I would treat all of their claims with caution.
Edit: This may not be a bad from thing from an instrumental rationality perspective. If you think that the problem you're working on is really important then you're more likely to put a good effort into solving it.