The speaker is successful in immortalizing his beloved. Part of this success is showing how his beloved is more "lovely" than a summer's day. Summer is too erratic, whereas his beloved is more temperate. Summer has "rough winds," is "too hot," or it's "lease hath all too short a date." Here, "temperate" means mild. And in the context of being eternal, temperate also means unchanging. Summer only lasts three months, while his beloved's "eternal summer shall not fade." So, how can his beloved's beauty and/or essence be immortal?
Since his beloved cannot literally survive death, nor can he be young and beautiful forever, the speaker intends to find another way to immortalize him. This immortalization has more to do with the overall effect of the poem itself than it does with the metaphors and comparisons between the summer and his beloved. The poem itself is what the speaker intends to immortalize. As an immortal tribute, the poem then gives immortal status to his beloved. The final two lines express this:
So long as men can breathe or eyes can see,
So long lives this, and this gives life to thee.
If the poem is immortal, so is his beloved. So, it is about poetic devices, but in a broader sense, it is about writing itself.