To me it seems pretty simple although I don't think I'm smarter then the authorities in about 40 countries.
The plane was considered airworthy before it was put into service, then there was an accident and as a result the FAA put in guidelines for updated software with a timeframe for Boeing to implement it.
Which additional evidence has been added to the situation as reason to ground the planes? Other then people's emotions going loose and being nervous, there hasn't been a single solid indication that this latest accident is related to the airworthiness of the aircraft.