I like the analysis, but I don't entirely agree. Whilst I doubt they'll disappear, I think Makkah might become the lesser counterpart to Jeddah (as in OTL), without the significant importance given to it by Islam. Those pilgrimages made the owner of Mekkah very wealthy - without it, the frankincense trade and overseas trade is better served by Jeddah, with Makkah as a Bedouin overland trade hub, allowing easy shipping to Jeddah. I think it can be seen as analogous to Canterbury & London. Medina has a likely similar fate with an alt-Yanbu.
Both Mekkah and Medina would be import for overland trade, but will play second fiddle to the coastal settlements as soon as decent shipping comes along. That in itself may well be how the rulers of Medina takes the lead from Mekka - building a port and taking trade from the Hedjaz and shipping it overseas.
Unless that have some very clever engineering that makes them better urban locations, in which case Jeddah might be the Ostia to Mekkahs Rome.![]()
This is correct. I do like the comparison between London and Makkah or with Rome.