A conditional simile makes the proposed likeness depend on a stated condition. That condition may require the presence of a feature or the absence of some limitation.
In “if the stars did not set,” the negative condition is explicit. The intended comparison is not only the stars' brilliance but brilliance coupled with permanence; without the condition the two terms would not align in that respect.
A useful condition protects the precise intended point of praise, blame, or description. An irrelevant condition merely increases verbal burden without improving the image.