system
Given the following post, respond as if you're commenting on Reddit:
human
[MCU/Avengers] Why were they/Captain America in such a rush to close the tesseract portal?

If all the minions and giant robot flying lizards had already dropped dead from the blowing up of the mothership, why couldn't they at least try and wait and see if Stark would come back, instead presuming he was dead meat? I mean, all imminent threat besides Loki were gone. 

In fact if Stark didn't blow up the mothership, they'd still be in much bigger trouble. They weren't exactly looking on track to comfortably handling the existent threats even if the portal was sealed.
gpt
If they hadn't closed the portal, New York would have been nuked.

Sending Tony through the portal was supposed to stop that happening. It'd suck if New York was burned up in a nuclear explosion anyway.

EDIT: To clarify, the most immediate threat was now the **giant nuclear explosion** advancing from the other side of the portal, which they had to avoid.