Bitcoin is not a perfect solution to the double-spending problem, so there is always a threat.

The security of Bitcoin depends on financial incentives, specifically the assumption that the participating parties try to maximize their financial gain and that they act rationally to achieve that goal. The security can fail in cases where that assumption does not apply. A double-spend via a 51% attack can never be dismissed as impossible.

Two plausible examples:

Bitcoin could be doomed if a country decides that Bitcoin is a threat to national security and is willing to spend whatever it takes to dominate the hash rate and destroy Bitcoin via double-spends and DOS.

In 2014, ghash.io achieved more than 50% of the hash rate and could have performed a double-spend. Normally, a pool would not do that because it would mean the end of the pool as miners would desert them. But suppose ghash.io decided that they wanted to become famous for being the only pool to successfully execute a double-spend via a 51% attack regardless of the consequences. In the end, it turns out they really had nothing to lose because they lost all of their miners anyway. They didn't do a 51% attack, but they could have and they might have been motivated.