As I understand Eliezer's current position, it is that the right thing to optimize the universe for is the set of things humans collectively value (aka "CEV(humanity)").
On this account the space of all possible optimizing systems (aka "AIs" or "AGIs") can be divided into two sets: those which optimize for CEV(humanity) (aka "Friendly AIs"), and those which optimize for something else (aka "Unfriendly AIs").
And Friendly AIs are the right thing to "side with", as you put it here, because CEV(humanity) is on this account the right thing to optimize for.
On this account, "why side with Friendly AI over Unfriendly?" is roughly equivalent to asking "why do the right thing?"
The survival of creatures like us is entirely beside the point. Maybe CEV(humanity) includes the survival of creatures like us and maybe it doesn't.
Now, you might ask, why is CEV(humanity) the right thing to optimize the universe for, as opposed to something else? To which I think Eliezer's reply is that this is simply what it means to be right; things are right insofar as they correspond to what humans collectively value.
Some people (myself among them) find this an unconvincing argument. That said, I don't think anyone has made a convincing argument that some specific other thing is better to optimize for, either.
To which I think Eliezer's reply is that this is simply what it means to be right; things are right insofar as they correspond to what humans collectively value.
No. The argument is more like that there's no source of complex value in the world besides humans, and writing complex values line by line would take thousands of years, so we are forced to use some combination and/or extrapolation of human values, whether we want to or not.
If it's worth saying, but not worth its own post, even in Discussion, it goes here.