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Short-term: Site event that cannot be fixed by a rollback
Some types of distributed system failures cannot be fixed by rolling back to a known good version of the software. If the system gets into a bad state it can often require manual intervention by experienced engineers to get the system back into a healthy state. This particular risk is high in the short-term with the recent departure of entire teams until they can be restaffed and regain the necessary experience.
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Medium-term: Bankruptcy / firesale due to lack of profitability
… due to advertisers fleeing the platform. Elon Musk gets bored trying to be the savior of Twitter after none of his ideas work out to make Twitter profitable and his personal brand craters, so he blames woke leftists and cancel culture for Twitter's demise as he abandons ship.
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Long-term: Regulatory consequences
… due to firing the entire board of directors and losing/firing entire teams responsible for legal/compliance/moderation issues. This can (more likely) cause Twitter to suffer massive fines that make it wildly unprofitable or (less likely) get shut down in various countries if they fail to comply for a long enough period of time. Twitter was already on thin regulatory ice even before the takeover.
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Long-term: A security breach
… due to Twitter being unable to patch or upgrade their software. In the short-term this is not an issue because they can likely backport fixes to whatever software they're using, but what will likely happen is that the expertise to upgrade software or push new code without taking down the site is gone and it will get more difficult to backport security fixes as time goes on. Eventually they'll be too slow to prevent or mitigate a security vulnerability that the site will get hosed.