The fact is, we know from previous experience and from Airbus admission that the fin is very sensitive to excessive sideloads.
In very extreme turbulence or in a loss of control situation, it is not unreasonable to expect such high loads on the vertical stabilizer.
It will be important to find out if the possible loss of the fin at altitude contributed to the accident or not.
Of course it could have remained attached until the airframe plunged in the water, but I find that option less likely. Time will tell.