" Up till a few years ago, the traditional way of managing data (in SQL-based RDBMSs) was considered a relic of the past, as these systems couldn't scale to cope with such a huge amount of data. "

I must have missed the boat on this one. I remember in 2010 there was a brief period of time where NoSQL was in fashion, but it rightfully died pretty quickly to a small set of specialized use cases. There have been some cases where big data systems have replaced more traditional rdbms systems, but now you can use SQL for those too (like Hive SQL).

SQL is the one skill that has not become obsolete in the course of my career. Frankly I've started relying on it more, because it never goes obsolete. Also it's fast as hell. When I first started my career I did C#, and the .net Framework 2 was fairly new. Since then WinForms, and WebForms have gone away. ORM's changed, Javascript changed. Then I moved to Ruby, and Python, and PHP. Those ecosystems have evolved too. But the one thing that I learned 15 years ago, that I still use every day is SQL.