There was no chance of AT&T shutting down GNU software; because, they didn't have any legal leg to stand on.
The legal issues simply hadn't been settled in court, but there was a very real risk that AT&T could have made both patent claims (they had some software patents already) and copyright claims (based on identifier names and interfaces) against the GNU project. Furthermore, many of the people contributing to GNU did have access to UNIX source code in principle, resulting in yet more ways in which AT&T could have challenged GNU. At the very least, they could have tied up GNU in legal knots for years. And whether the GPL itself would hold up in court was yet another legal uncertainty.
The GNU project has always lived under legal clouds and threats; that just comes with the territory.