As the layout block contains the drawing sheet, the number, the viewports, it's the presentable part of the drawing.

Maybe QCAD could be able to save on the dxf the "last state" of the drawing. If I saved on the Model and closed, it would open on Model... and if my last save was on the Layout"n", it would open on Layout "n".

At least on my work area, when opening a finished drawing, it's the ideal for it to be opened in a Layout (if it was the active block while saving).