A foreign government being able to disable your communication whenever they want to is already a threat to national security

Well... yes it would be, but even though we're talking about US-based companies, the servers aren't all located within US borders, they're spread around the globe. Maybe the US government could force Amazon data centers within the US to stop carrying Signal data... though that in itself would be a logistical mess and I'm not sure how they would justify it legally. Even the current administration loses court battles and their actions get reversed, and at that point they'd be contending with the legal teams of both Signal and Amazon, plus probably the ACLU and EFF. Considering how few competent lawyers are left in the Trump administration, I doubt they would win that court battle.

For the servers outside the US, not only would it be a logistical and legal nightmare but I'm also not sure how it would be done in a technical sense, at least not all at once or in a time frame of less than a month. It would be very difficult to accomplish actually shutting down Signal's operations in any practical way. You would have to exercise legal authority or physical force over more than half the data centers on the planet (AWS, Google, Microsoft).