There are definitely problems for which RDF is the right answer, and trying to use a relational database for those problems is painful at best. For example, while you *could* do a semantic web thing using a relational database, you wouldn't want to.
On the other hand, there are definitely tasks for which RDF is wildly unsuitable. Just at the moment, I'm working on an inventory management application. Again, you could do such a thing in RDF, but you'd very quickly regret it.
Any database is a model of your real-world data. All models require reductionism, and thus the model format you use should be based on what it is you want to do with the data you have. Claims that one or another data model form is "more realistic" or "more natural" than another are specious. All models are equally artificial.