You mean other than being a convenient way to close off the story?
It sort of argues for the whole thing to be a hallucination: Dick knew his trips were coming to an end so he hallucinated an ending. At first I thought that if it was made up then he'd have made up a happy ending for Isolda, but maybe he didn't want her to have a happy ending without him.
But if we accept the effect of the drug as real - which, within the story, I did - then it seemed possible that maybe whatever it was in formula A and B that tied us to Roger (and jumped us to a certain age rather than growing up with him), that maybe formula C did this with Robbie. (Thinking of a polyjuice-like potion with a dash of hair to tie us to a particular person - but something not so flippant as that sounds.)
Bookpossum's
first post on this thread includes a quote suggesting that du Maurier liked irresolution, and perhaps that's what we're seeing here: a twist calculated to open such questions in the reader's mind.