I will assume with your post that you see AI as something that might endanger humanity.

You were talking about S-curves, but then you completely ignored them, in the last 200 years life expectancy was increasing at a very good pace, just like an S-curve, it started slow, then it got its vertical take off, and eventually it started to slow down. This doesn't mean that it is the end game though, every S-curve works on top of a much bigger S-Curve, so just because you see this S-curve ending, it doesn't mean a new one isn't starting. And I will argue that just as we have a S-curve ending with the cure of almost all diseases, reaching the limit of our biological bodies, we have indeed one that is starting, as AI starts to solve aging, be it with genetic engineering, artificial organs or other methods.

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I don't see AI/Robots as the next stage of evolution, but a tool humans will use to enhance themselves and evolve into a new kind of lifeform. Evolution doesn't occur in phases like we imagine, it is very incremental, some specie suffers a mutation an then another mutation and another and eventually it starts to become distinct from its ancestors, but that takes a lot of time and it is impossible to pin point when exactly did it got distinct enough to become a new specie. The same way, we just won't wake up some day and all of a sudden we have an AI that will end humanity. It will be very fast (compared to normal evolution), but it will have a somewhat incremental trajectory. Right now we have tools like Stable Diffusion, ChatGPT, in the next few years these kind of tools will increase substantially, but the keyword here is "tools". For the foreseeable future, these tools will be in complete control of human beings. So even if you believe AI will one day gain sentience, by then we will have an arsenal of AI tools at our disposal, from text and image, to law, engineering and medical, humans will greatly evolve from those. BCIs is the next step from smartphones, it will further increase the speed of communication and access to information with the help of these tools. With this I would argue that humanity is already in the beginning phase of its transformation, the start of a new S-curve.

And I will argue that humans will not create something they will have no control off. New AIs will always be at one button away of being switched off. Here the Alignment Problem comes into play, it is important to develop these AIs in a safe environment and I'm sure as soon as humans with the help of AI tools, realize that AIs are starting to develop dangerous tendencies, security will become much tighter.

To end I believe there will be indeed a new kind of specie, but it won't be AI/Robots, but an evolution of humans. Now, not every human will want to embrace these changes, some might indeed want to keep being fully human, rejecting BCIs, genetic engineering or other forms of new advancements we might have, you can see examples of this in the Anti-Vax movement, recently there has been a lot of hate for AI generated content as well, so in the future humanity might go 2 separate ways, it will be interesting times ahead for sure.