I would contest they are generally necessary.
Applications of parametric CAD are many. Can a jeweller designing a parametric ring do without them? Probably.
That said, you can e.g. calculate derivatives of the SDF and, simply speaking, when they 'flip' you know you found an edge or corner.
You can project onto the SDF along a plane or line and 'record' the intersection, use it as input in your model etc.
Similarly, you can treat an area bounded by such edges as a face etc.
Just because there is nothing out there yet doesn't mean it can't be done.
I also predict a great future for SDF based CAD for the simple reason that manufacturing methods mostly used for prototyping now (3D printing) will become more common for final pieces.
And there is no need for an explicit representation (higher order surfaces or polygon meshes) of the shape to get tool paths (often derived from 2D bitmap slices) for these methods.
One thing that probably won't change is the need for things which we design to be made of multiple parts which must be assembled together. For any serious application this requires specifying geometric dimensioning and tolerancing information, which ideally should be tied to the topological entities in question.
I'd argue that this information is becoming even more important with the rise of 3d printing and other digital manufacturing tools. If this information can be included in the CAD data, the manufacturing tool might automatically determine how best to manufacture the part to meet the required tolerances.
Topological information is also useful for analysis. We may wish to apply a load or boundary condition only on a face.
That said, you can e.g. calculate derivatives of the SDF and, simply speaking, when they 'flip' you know you found an edge or corner. You can project onto the SDF along a plane or line and 'record' the intersection, use it as input in your model etc.
Similarly, you can treat an area bounded by such edges as a face etc.
Just because there is nothing out there yet doesn't mean it can't be done.
I also predict a great future for SDF based CAD for the simple reason that manufacturing methods mostly used for prototyping now (3D printing) will become more common for final pieces.
And there is no need for an explicit representation (higher order surfaces or polygon meshes) of the shape to get tool paths (often derived from 2D bitmap slices) for these methods.