My contemporary interpretation:
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We hope for more beautiful things
So in that way the height of beauty remains alive forever.
Although he who matures must eventually die,
His own child would perhaps share a similar beauty.
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From fairest creatures we desire increase
--We hope (desire) for more (increase) beautiful things (from fairest creatures)
That thereby beauty’s rose might never die
--So in that way (thereby) the height of beauty (the rose) remains alive forever (might never die)
But as the riper should by time decease
--Although he who matures (the riper) must eventually die (should by time decease),
His tender heir might bear his memory.
--His own child (tender heir) would perhaps (might) share of similar beauty (bear his memory).