1. AIs have made rapid progress in approaching and often surpassing human abilities in many areas.
2. The fact that AIs have some inherent scalability, speed, cost, reliability and compliance advantages over humans means that many undesirable things that could previously not be done at all or at least not done at scale are becoming both feasible and cost-effective. Examples would include 24/7 surveillance with social desirability scoring based on a precise ideological and psychological profile derived from a comprehensive record of interactions, fine-tuned mass manipulation and large scale plausible falsification of the historical record. Given the general rise of authoritarianism, this is pretty worrying.
3. On the other hand the rapid progress and enormous investment we've been seeing makes it very plausible that before too long we will, in fact, see AIs that outperform humans on most tasks.
4. AIs that are much smarter than any human pose even graver dangers.
5. Even if there is a general agreement that AIs pose grave or even existential risks, states, organizations and individuals will are all incentivized to still seek to improve their own AI capabilities, as doing so provides an enormous competitive advantage.
6. There is a danger of a rapid self-improvement feedback loop. Humans can reproduce, learn new and significantly improve existing skills, as well as pass skills on to others via teaching. But there are fundamental limits on speed and scale for all of these, whereas it's not obvious at all how an AI that has reached super-human level intelligence would be fundamentally prevented from rapidly improving itself further, or produce millions of "offspring" that can collaborate and skill-exchange extremely efficiently. Furthermore, since AIs can operate at completely different time scales than humans, this all could happen extremely rapidly, and such a system might very quickly become much more powerful than humanity and the rest of AIs combined.
I think you only have to subscribe a small subset of these (say 1.&2.) to conclude that "AI is an uniquely powerful and thus uniquely dangerous technology" obviously follows.
For the stronger claim of existential risk, have you read the lesswrong link posted elsewhere in this discussion?
https://www.lesswrong.com/posts/uMQ3cqWDPHhjtiesc/agi-ruin-a... ?