This is a fair suggestion, although predictions that the Mongols would race through North Africa until they reach Morocco if Ain Jalut went the other way ignore the giant expanse of nothing between Alexandria and Benghazi. Mongol armies had tons and tons of horses and they weren't about galloping across open desert with nothing to eat. The Mongols would've taken Alexandria, Cairo, and the rest of the Nile Valley and called it quits with Egypt.
Submitting for consideration the Battle of Badr. If the Meccans killed Muhammad at Badr, no Islam. No Islam, no Arab Conquests. No Islam, no separating the Levant and North Africa from European civilization and the two sides of the Mediterranean would be far more similar. No Islamization of Persian and later Indian civilization. Central Asia remains Buddhist (or Manichean I guess). No Islam changes EVERYTHING. And the Battle of Badr was tiny. Just a few hundred people on each side.
EDIT: I didn't realize this was a bump, sorry.
Wait am I not allowed to bump threads especially in a smaller board like this? I understand why necroes aren't great but history hasn't changed.