Software development is an industry still in it’s infancy. Technologies are continually evolving, and trend come and go. What’s hot today can quickly get replace by something new tomorrow.
It’s foolish to think that you can learn one thing and then be done with your learning and continue in your software development career for the next couple of decades without learning anything new. The past decades have already shown that what’s the hot in-demand tech skill now is likely to be in demand several years from now. That said, there have been programming languages that have stood the test of time: COBOL. C, and Java are some examples.
In order to stay relevant, it’s important that you spend time to keep your skills up to date. Make a commitment to learn something new (a new language, a new framework) every several months. As you gain more experience you’ll start to make better decisions on new trends that you think will stay around for longer, or those which seem like they’re likely to disappear within a few years.
Bottom line: expect and and plan to keep your tech skills up to date.