It seems that new paradigms like R and RAM-only key values system are just simpler, faster and cheaper in programmer time. Loading everything in RAM and do a search seems like a huge saving of time and just works for most cases. Usually if you have more complex needs, you need to adapt your solution, and databases don't seem like a silver bullet.
Database queries seem like a solution to the problem of storing data on disk when RAM was too expensive. So today it's still used as some sort of standard, but when you can have 16GB of ram, I think you better teach yourself what sort of algorithm and data structures a database use to be faster, use the ones you like and need and solve your problem case by case.
The example of how reddit store its data is pretty demonstrative that ultimately, you should not let a database system do all the work. Databases are just a file format to me, but the way it tries to work for you at a lower level will respond to basic cases, but when you increase complexity it's not relevant anymore.
Especially today when you have big data and machine learning, everyone should just learn to understand data manipulation. Not saying to teach yourself C all over again, but having a decent idea of the math of what indexing is really about. Forcing yourself to use a database because the company always used it isn't appealing to me.
It's like one of those things when a programmer had an idea which is based on a constraint, everyone starts using it, several products are made, but nobody really remember the original idea of the inventor of that paradigm.