If you're being serious- you earnestly believe people who pirate a Japanese game and apply a translation to it would actually buy an official English version rather than pirating it the same way they would the original version? I don't feel like the existence of a fan translation gives someone more or less of a tendency to not buy a game.
As Para mentioned, Kagura has also adopted plenty of titles with existing unofficial translations, and other circles adopt fan translations as their official translation to expand their audience. There's not really much of an argument for fan translations hurting sales, even bad ones (ie: MTLs). I won't argue with you on MTLs being garbage, because they are, but their mere existence isn't "ruining" games any more than the existence of text hookers.