I agree, I don't think the purpose of sci-fi is to predict the future. The future is just impossible to predict due to the myriad factors, variables, unknown unknowns, and second-order+ degree effects an action can have.
The purpose of sci-fi IMO is moreso to:
1. Provide an entertaining story/narrative with technology as the main focus of the world and characters' actions
2. Define a set of concepts to help you think about technology and its possible effects on humans and the world
3. Nudge people to think about what kind of future they would want or not want and how they can use or control technology to achieve that
I do think predictions are an unavoidable side-effect of writing science fiction, but those predictions are almost always in service to those purposes you listed. Stephenson must think nanotech book AI is more plausible as a form idealized childhood education than say, magic fairies. But in the end he relies on our suspension of disbelief to get on past that to the big questions of nurture vs nature and the quality of human intelligence.
The purpose of sci-fi IMO is moreso to:
1. Provide an entertaining story/narrative with technology as the main focus of the world and characters' actions
2. Define a set of concepts to help you think about technology and its possible effects on humans and the world
3. Nudge people to think about what kind of future they would want or not want and how they can use or control technology to achieve that
Here's Ken Liu talking about the purpose of sci-fi: https://www.youtube.com/watch?v=5knkpmxXu-k