I think there's a certain fallacy in thinking that any appearance of previously seen tropes - such as elves, dwarevs, magic, wizards, dragons- is an exact copy of previous iterrations and therefore completely worthless as a work. By that logic, there should not be any dragons, manticores, any angels or demons, any creature at all that come from another source, since they have been seen and done to death over and over. Why do we still have dragons? Because they are easy to relate to. They represent something which anyone can recognize. You could invent a giant horrible monster the heroes must defeat... or you could have them fight a dragon. The later may sound lazy and devoid of creativity, but it draws in the audience as everyone knows what a dragon is capable of. They know what the stakes are, while an invented monster is an unkown. You can explain what the monster is, of course; you can detail out all of its biology, its desires, its intelligence. But all that takes time, and effort, and time and effort on the part of the reader to integrate all that information. Conventions exist because they let you skip all the messy business of learning the workings of the world, and go right to the story at hand.

As you pointed out before, we need to be careful not to become mired in worldbuilding, which I agree with because if it takes too long the whole project will die a slow death as people get bored and leave, and the exponentially increasing complexity makes it difficult to keep track of other people's creations.