But isn't that the idea behind Rails? Opinionated defaults with a config to change the setting somewhere?
Most of the things that Rails is "opinionated" about are far more arbitrary than this, like whether the database primary key should be “id”, “postId”, “posts_id”, or “pid”.
To my eye, at least, it isn't a matter of being arbitrary, but rather not having an answer.
PK names don't matter to the machine, so long as everything agrees and modulo illegal values. Humans get massively opinionated, but the machine doesn't care.
Compare with: How long should a query take to run? No, I'm not going to tell you what kind of query, or the size of the dataset, or the hardware at the bottom of the stack, or what the app is actually doing.
It is like asking what specific clothes an arbitrary person should wear.
Also this stuff is all heavily Postgres specific; I think @nateberkopec's estimate of 300 gems is probably too low, even if you limit it to only say the top five most popular datastores.