Even if it expands at the speed of light, it would likely be outside of our observable universe though, right?
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.
If I'm understanding you correctly, you're arguing that since we've had 15 billion years so far with no collapse, then odds are great that we've got billions more to go. My point is that if the bubble collapse is not instantaneous, then we don't actually know there's been no collapse so far.