The POD is that Salman of Persia never converts to Islam, and never gives the idea of digging a trench around Mecca.
In March 627, the Muslims, tired of hiding in the city of Medina since being driven from Mecca, meet the army of the Quraysh and their allies in open battle, and lose. Muhammed is killed in the battle, Medina is looted and the remaining Muslims, led by Ali, flee to Ethiopia.
Over the next several years, the old order is reimposed over the Arabian peninsula, with the Quraysh tribe even more powerful than before, controlling Medina as well as Mecca. The leading tribes are just as corrupt as before, if not more, with virtually no protection for the poor or for younger children of powerful clans. The Quraysh buddy up with the Persians, who are still smarting from their defeat from the Byzantines. Heraclius, for his part, is consolidating his gains for the moment; in Ethiopia, the Meccan's desecration of the church in Sanaa is not forgotten, and the Christian king makes friends with Ali, who has established himself as the leader of the Muslim emigrees (most of those survivors who would be Sunnis in OTL decide to leave to establish their own community, whether by land to central Africa or by sea to somewhere else).
Around 632-635, discontent boils over in the Arabian peninsula, and revolts against the most powerful tribes break out. The Ethiopian Negus decides to take this opportunity: he arms Ali and the rest of the exiles, and they land in Arabia, marching towards Mecca. Ali captures Mecca, and scourges the idols from the Shrine of the Qa'ba. In exchange, Ethiopia regains Yemen.
So now we have a Muslim state centered in Mecca, backed by Ethiopia, a pagan state centered in Medina backed by Persia, and things are probably going to flare into all out war soon.
So, how plausible is this? Please point out any errors I made.