Regarding myself I often use photomontage to inspire what I do. But I always recreated/change it so much that it is unrecognizable. But the thing is, at one stage in the development ( before it is finished as a preliminary sketch or preparatory work), it may still be recognizable, so it was useful even if the final work look nothing like it.

Most photographers have a too high opinion of themselves. The thing is, very few photographs count as a real work of art in the legal sense. You have yourself shown, if not enough originality is present in the original work, you can not sue anyone for making derivative compositions from it.

There is a famous high profile court case in USA, where a museum tried to claim that their photographs of the paintings in the museum, in their own sense was a work of art and should have copyright protection. But they lost. It was ruled that the photographs did not contained enough originality.

So, photographs do not enjoy the same protection as a work of art. Except in the few cases that the photograph itself counts as a work of art. Very few Photographs do.