there are many doctrinal authorities on record in every large strain of western monotheism against cruelty to animals

Which means that many doctrinal authorities are capable of making stuff up.

While most religions' tenets require some interpretation of their holy books, there are degrees of this. Some claims made by religions come from their holy books in a fairly direct and straightforward way. Others are claimed to come from their holy books but in fact are the result of contrived interpretation. Religious animal cruelty laws fall in the second category. The holy books do not support laws about animal cruelty in the same way that they support "thou shalt not commit adultery".

Furthermore, even those contrived laws don't generally claim it's cruel to eat animals. Bringing up the fact that religions oppose animal cruelty is like pointing out that every religion and culture has rules about sexual immorality, and therefore we should oppose some particular type of sexual immorality that you don't like.

they are in competition for resources with neighboring tribes, they would kill neighbors whenever they thought they could get away with it if they attached zero utility to these people's survival.

During much of history, most cultures that knew Jews attached zero or negative utility to them, but pogroms only happened every so often. They didn't just kill all the Jews until the Nazi era.

What we know from the psych side is that empathy appears to be basic in humans.

Anthromorphizing is also pretty basic to humans; that's why the Eliza program convinces people.

But it wouldn't surprise me terribly if the "expanding circle of concern" eventually encompassed or re-encompassed things like trees and rivers.

But you're not following the implications of this. The idea that primitive cultures respect the spirit of animals was brought up to show that taking the well-being of animals into account is normal. If the same primitive people respect the spirit of things whose well-being we clearly should not take into account, such as vegetables, it doesn't support the point you brought it up to support.