So, how should you invest your time as a developer? Should you focus on getting better at your current tech stack, the one you're using right now, or should you learn new languages and technologies? To answer this question, I will use the concept of anti-fragility from Nasim Taleb. Nasim, it's a former stock trader. What he discovered is that if you don't take any risks, you only stick to what you know, you do risk being disrupted when things change.
And in software, they change all the time. On the other side, if you're always jumping from shiny framework to shiny framework, you risk not developing any real, deep expertise in any of them. The solution here is to become an anti-fragile developer. Focus 95% of your time on getting better at what you already know. For example, the JavaScript stack, if you're a JavaScript developer. And the 5% remaining, focus it on an exotic fancy technology like Elixir or Go.
In this way, you make sure that your core skills are strong, but also benefit from all the new shiny things out there.