Fail is a different concept from Success. Success is the opposite of Mistake. Mistake is related to Screw. 
Could is etymologically related to Will. Will is related to Future. 
Fail <--DistinctFrom--> Success <--Antonym--> Mistake <--RelatedTo--> Screw 
Could --EtymologicallyRelatedTo--> Will <--RelatedTo--> Future 