People don't have time (or even mental capability / even more time) to learn CAD but they have a task at hand which they want done. If AI can generate a part which is "good enough" for the user at a cost that the user considers reasonable (I.e. zero) then AI gets used. If a part fails thats rarely a concern because these parts are just 3d printed and likely cost single digit cents in filament. User can always print a new one.

Right, but the thing that I think sort of militates against this scenario is this:

If a non-CAD person imagines a part that they need, the chances are very high indeed that either it isn't what they need, can't be made robustly or sensibly, or if it is, it already exists in some meaningful way that can be adapted, because one of tens of millions of other people already wanted it.

This scenario — needing something very specific that can be described only in words and nobody else has ever needed before — is almost imaginary. Most stuff just isn't like that.

Now… if someone were to design an AI CAD package that could remix objects that exist and combine them, perhaps with a GUI tool that allowed a user to identify attachment points, maybe that would be interesting. But a lot of that has been solved in non-AI GUIs already.

Text-to-CAD just isn't as good an idea as it seems. You are not going to be able to magic up some novel invention from a text description without a significant amount of domain knowledge of the words needed to describe a part, and that domain knowledge usually comes from learning CAD!

(Bit like getting a coding agent to write really good code)

And simple parts, sorry, just don't need much CAD skill. Get a CAD package on a free plan, learn enough to make your thing. You will get so much more out of that and it will be easier to make changes. It'll probably have standard tools for adding threads, even.

Even image-to-CAD for a functional part requires a significant amount of understanding of what to photograph and from which angles, etc.