Does a programming language have to live vibrantly for 3 decades to be "good"? I don't think so. Use it while it's good, then move on to or create the next good thing.
The longer it lives, the more your investment in learning thoroughly it pays off. But languages also have a tendency to accumulate features until taking on a cancer-ridden look. I probably stick to a given "main workhorse" language for about a decade on average. So I would agree that if a language gets a decade of being good, it is about as good as you can expect.