The first 3 quatrains all use the verb "shall", which is future tense. In the last line, Shakespeare changes to present tense. Why would he do that?
I have read Shakespeare's Sonnet 55 many times over the years, but it was only on reading your question that the thought occurred to me that the person the speaker is addressing (presumably a young man being addressed by Shakespeare himself) might already be dead.As you observe, the word "shall" appears over and over again and is meant to foretell the future--but when Shakespeare switches to the present tense he well may be implying that someone who has died isonly living in his sonnet. This suggests that this sonnet differs from many of the others in being an elegy, which could explain why it starts off with references to marble memorials and gilded monuments to the dead. It is as if Shakespeare is offering his own memorial to some recently deceased friend. If the person being addressed in the sonnet isnot already dead, then this sonnet might seem to contain rather gruesome and morbid sentiments. After all, Shakespeare would seem to be predicting the young man's death and burial. How would this make the young man feel when he read it? The tone is entirely different from the ones in which the speaker is encouraging a young man to get married and have babies--as in Sonnet 1 which begins, "From fairest creatures we desire increase." The closing couplet might be interpreted as saying that until Judgment Day youonly live in this sonnet and not that you will continue to live in this sonnetafter you die.