Sif isn't that iconic or even well known anyway. It won't cut into Alexander being seen as Wonder Woman.
While this is true and not up for debate it is amazing that Marvel has managed to make Sif at least a known quantity. Same for Black Widow to the larger moviegoing populace if nothing else.
At some point when Sif is recast, especially after a third turn in a Thor 3 no doubt, down the road Jaime will have made Sif a character with some broad fandom about her that she's never had in 50yrs really.