Its very unlikely to happen anytime soon because Al Shabaab is Al Qaeda-affiliated transnational outfit whose vision is NOT to takeover & rule Somalia but to engage a global war against west & their ‘local client states’
If Al Shabaab had a Somalia-only agenda and if they were NOT Al-Qaeda wing in East Africa, the west may be open to let AS rule Somalia — as they now did with Taliban in Afghanistan & as they did 2009 in Somalia with the Islamic Union Courts, which AS was imbedded with. then.
International community’s discontent with Somalia’s elite has not YET reached a boiling point disoriented priorities & gung-ho like posture to Somali stakeholders & international partners has created Somalia fatigue / pessimism which could make them flip.
Since Al Shabaab is not as strong as the Taliban & are not Somalia centric & since the cost of holding them at bay is not as exorbitant as the Afghan, and because of AS’ Al Qaeda affiliation, Somalia may not fall to AS anytime soon & the west is not YET there to pull the plug
but if Somalia’s troop contributing countries such as Uganda gets tired of the Somali political elite’s disorientation and or if the situation of Ethiopia gets out of hand, then, the Somalia situation may get a bit dicey because the Somali government is too weak to fend for itself & if the African Union troops were to withdraw, the gov would probably loose control of the towns surrounded by Al Shabaab, including Mogadishu.