I am in full agreement with you that if it is just Somalis vs Alshabab, the likliehood of Alshabab winning is closer to 80% than to 25%. Most Somalis greatly underestimate how powerful Alshabab is in comparison with other Somali forces.
The reason I have it at 25% is that I don’t think Kenya and Ethiopia will allow for the complete collapse of the FGS. They can both easily afford to place 15,000 troops in the way of Alshabab and keep the FGS going. I am assuming that the world won’t allow a UN member state to be ruled by the local Al Qaeda franchise.
Amisom will stay as long as the west pays its salary. Kenya won't be able to afford to stay in somalia if amisom wast the leave and neither would ethiopia.
So you hoping kenya forever to save your uncle madoobe in kismaayo. Let me break it to you the moment they leave, kismaayo will fall and most likely to Al shabaab. That's because "jubbaland" has not been build from the ground up it was build as a buffer zone by ethiopia and specially kenya. Jubbaland does not represent the people of jubbaland and same argument can be said for the bunker government in xamar.