Soooo, you think good writers just decide to phone it in when casting (a completely different production group) makes their casting choices? Even though much of the writing usually comes well before casting? Seems like a stretch…
Even ignoring that little detail, this is in the context of fantasy races which have no connection to our real world races. The character of Arondil is a Sylvanas elf, just like Tauriel. Whether he’s played by a black Englishman, white New Zealander, or (in this case) a Puerto Rican doesn’t really have any bearing on how he’s written. He’s just an elf.
The only example that really comes to mind is the Ghostbusters movie that tried to bank on its cast ad libbing through the movie without a decent script, but at the same time comedians like Will Ferrell and Zack Galifianakis have had their fair share of movies bomb by doing the same. Lazy writers are just lazy regardless of what gender/race they’re writing for.