a. You will write poems as lovers can.
b. You will write poems as only lovers can.
(You will write poems the way (only) lovers can)
c. Your love will fade away as certain memories can.
d. Your love will fade away as only certain memories can.
(Your love will fade away the way (only) certain memories can fade away.)
Are the above sentences grammatically correct?
I have rarely heard 'as' used this way with 'can'. I can only think of: "He does it as only he can." and "He did it as he could." I think those are fine.