Imagine if it had been as severe as the
Peshtigo fire that night.
The air was no longer fit to breathe, full as it was of sand, dust, ashes, cinders, sparks, smoke, and fire. It was almost impossible to keep one's eyes unclosed, to distinguish the road, or to recognize people, though the way was crowded with pedestrians, as well as vehicles crossing and crashing against each other in the general flight. Some were hastening toward the river, others from it, whilst all were struggling alike in the grasp of the hurricane. A thousand discordant deafening noises rose on the air together. The neighing of horses, falling of chimneys, crashing of uprooted trees, roaring and whistling of the wind, crackling of fire as it ran with lightning-like rapidity from house to house—all sounds were there save that of human voice. People seemed stricken dumb by terror. They jostled each other without exchanging look, word, or counsel. The silence of the tomb reigned among the living; nature alone lifted up its voice and spoke.
The town was effectively destroyed and between 1,500 and 3,000 people died there.
If the conditions had been right and a firestorm occurred in Chicago thousands would have died and the devastation would have been on the order of Hamburg in 1944. More sustained winds, to fan and spread the fire, and an absence of the rain that began late on 09OCT1871, would have been necessary.
However the city would almost certainly have been rebuilt; historically the reconstruction began almost immediately, financed by business owners and (of course) land speculators. Even if the stockyards and railway infrastructure had been destroyed I'd expect rebuilding to happen, albeit more slowly.