"Big Data" is horse shit. It's code for "stuff it all in a pile and maybe someday we can get some of it out, but not in any order that makes sense, and not in any reliable fashion".
The problem with arguments like this is that it overlooks the fact that there is a lot of value in data that doesn't need ordering in advance, and where any missing gaps in data are not critical failures. Not all data can or should be stored in such systems, but for that which can, there are much better ways to store it than RDBMS. RDBMS isn't going away, but it's certainly been commoditised. In time, so will Big Data type storage, but until then it's a good thing to make good money on if you are starting out in this field precisely because it is relatively new, and also is not going away.