Grunt is actually not still fine to use. I've been burnt by a lot of plugins coming out for Gulp only and now for webpack only. What the GP says is true: if you start using any of these JS tools prepare to switch them out every 6 months.
As a full stack JavaScript developer, the tools I used over 6 months ago are also the tools I use today. Only difference is I left Angular for React. But my build tools are still the same. I use Gulp, SASS and Webpack.
You're talking out of your ass if you actually believe something can get to the popularity of Gulp or Webpack and then instantly be "old news" and "not worth using" within 6 months.
Maybe I'm reading this too literally, but personally I'm totally cool with using a tool for 2 years and then transitioning to another. 2 years is already a lot of good value.