Copley's Cap hit for the Caps is only $25K. I believe when he signed, Sammy was still in Russia*, Vanny had finished his 3rd full season in Hershey and his NHL prospects were not yet clear. So at the time, his signing was not purely fodder for Seattle. He was signed to be backup to Holtby. He had finished his 2018/19 season with 27 NHL games, 2.90 and .905, so he had shown competency as an NHL backup. It just happens that Sammy has advanced above him and so too has Vanny since then. Not everything is a conspiracy or a stupid move, sometimes just plans change based on development of other players.
*Correction, when Copley signed, Sammy had just finished his 1st AHL season where he came over mid year and it was a mixed bag. 2.70 and .898, but if I recall correctly it was a dreadful slow start then warmed up nicely at the end, so at the time Sammy's NHL prospects were not yet clear. Also the alternative to signing Copley to a modest backup contract was to let him walk for nothing... Right now we know what he is, AHL starter, NHL backup. For $25K to the Caps cap.