perhaps naive newbie question about book III ending

[Dale]

No: HPFGUIDX 176106

When Hermione and Harry went back in time, why didn't they just tell
Remus to take his potion.  Then the book would have ended more happily
with Scabbers being put into custody, Black being cleared and Harry
living with him for the summer.  Am I missing something?  Of course
the Hippogriff would still be dead.

Thanks in advance, I'm new at the books and the group thing.

Dale