Do you know, that… ?

SOLIDWORKS has some limitations in sheet metal. Well, the cut has to be done like this, to the ends were joined by a straight line.

In practice, that's what it means, that the tapered plate cannot be cut other than parallel to the axis. If you create such a cut, then projecting the sketch onto the face of the cone will result non-planar edge.

It is impossible to replace such a model with sheet metal!

But… if I use this or a similar model and create sketches based on the edges – this can be used Lofted bends for sheets. In this case, it is possible to make a expandable sheet, but the effect is different than intended.

In sum – the cut must be parallel to the axis of the cone.