As others have noted, absolute safety is a myth - nothing in life is completely safe.
For certification purposes, there is a definition of safe:
The probability of a catastrophic outcome is ~ 10-9/flight hour (or less - less is always better).
The miss was that no one at Boeing or the FAA (or EASA) identified MCAS as being a flight critical system that needed to meet that 10-9 requirement.
I'm not privy to everything the 'fix' entails, but 10-9/hr. is the standard it will need to meet to be determined safe.