Step 1: Analyze the common usage and literary reference.
The phrase, when considered without literary context, suggests a direct progression or inevitability. Thus, "straight" implies a direct, unaltered path leading to an outcome.
Step 2: Select the appropriate word that enhances the meaning.
Although "but" is a famous part of Thomas Gray's poem "Elegy Written in a Country Churchyard," in a straightforward fill-the-blank without that specific literary context, "straight" could also be appropriate as it emphasizes the inevitability and directness of the outcome (death) following glory.