Well, if the theory is correct, it would not be observable until the moment we suddenly all died.

But the important difference between "stable" and "metastable" is that metastable systems spontaneously collapse, whereas stable systems don't. According to the theory, we could be at a metastable vacuum energy state, which means that we could spontaneously drop to a lower energy state at any time. The difference between "metastable" and "unstable" is really just the time span involved.