(Sorry, should have said Tuvix.)
No Doctor means no cure.
The Doctor is arguably the finest scientific mind in the Federation.
If you erase the procedure from his database. Or roll his personality back to a starting character, the "cure" is going to be lost and given time a much less proficient version of Doctor might need 60 days instead of 30 days to redo the the experiment and figure out how to separate Tuvix.
However if the Doctor gets too close to figuring out how to kill Tuvix again, Tuvix is either going to roll hm back again or stop fucking about and out right kill him, withwashing the medical database into a total blank slate.
"It was him or me."
And when "him" is a hologram it's really not any sort of decision at all.