Acceptable losses.
Total extinction of 15-20 races every 50.000 years is better than total extinction of all organic life.
Saw no evidence for that. There is no peer review or anything besides the Catalyst telling you that. He might be convinced it´s the truth, but actually there´s nothing showing us, that it´s not a runtime error or other bug.
Anyways there are better ways than that to ensure that organics wouldn´t be eliminated by their creations, called active policing. One of the more likely hypotheses about the singularity is, that there will be only one ASI in existence. The kind of AI which self improves exponentially until it moves far beyond our puny brains to keep up with. Unless there is some kind of blockade, like hardware constraints, the first one has a longer time to refine itself and can swat the other AI kids at its own leisure.
So another ASI shows up and it´s malicious? Grill it. Considering the age difference and processing power at its disposal it would be child´s play.
No they are not. They are both synthetic and organic.
from wikia: The Reapers are a highly-advanced machine race of synthetic-organic starships
A Reaper is essentially "billions of organic minds, uploaded and conjoined within immortal machine bodies" (Legion in ME2)
That´s copied from the codex. I find that highly doubtful. It´s really a stretch to call something organic because it is made from stuff that was organic a long time ago and then heavily processed. Quite a lot of these Reapers are older than crude oil or coal. No one would file a thing made out of plastics as something organic. It´s probably not a really fitting analogy but whatever the Reapers do with that organic goo, it´s heavily processed.
Legion´s statement is actually interesting, it´s too sad that it´s hidden in a rather hard to get conversation which you can easily miss. Anyways, every Reaper was rather uniform in behaviour towards organic races. At least in the realm of "we have to kill all of them." No idea what these minds are, it seems that they don´t have much say in the whole affair. And they put former organic minds, thought patterns or only simple memories in a giant network which is probably not running on individual brains. Whatever your mind is, it probably won´t stay recognizably the same when uploaded into a giant server. It happens that people change their behaviour and personally drastically when parts of the brain are damaged. What happens when you replace the whole mushy thing?
a. "his race of synthetics"
reapers are not synthetics. They are hybrids, created by harvesting organic races. Their mind, consciousness, "soul", is largely organic. They are not "ordinary" synthetcis: they are something more and something different, a primitive example of synthesis. To them, the "conflict rule" (synthtetics will always try to destroy organics) doesn't not apply.
As I explained previously, I don´t think that you can cal them organic anymore. Anything organic inthem has been so heavily altered, it´s synthetic. You can disagree of course.
But even if we agree with the the whole thing, it sounds like someone pulled off some weird legalese. The Catalyst talks about this whole synthetic against organic thing like it´s some kinfd of natural law. This explanation sounds like a loophole, a lawyer invented. ohoh, look how clever we are, we are thechnically not synthetics. Ah ok, sure.
And whatever the Reapers are, the Catalyst is an AI, why isn´t the galaxy dead yet? His whole "I am speial" talk is like trying to weasel out of an uncomfortable question. How about the created will always rebel against their creators? The Catalyst created the Reapers. Another special exemption it seems. So are there any more like "actually this is just bonkers applicable to the Leviathan cycle only, because the galactic masterminds were a bunch of control freaks?"
He SAID that "Without us to stop it, synthetics would destroy ALL organics." Plain and simple.
Advanced organic create synthetics -> synthetics rebel against creators -> synhtetics destroy all organics
How do you stop this?
Why should they? It´s based on ascribing human thought patterns to ASI. They went the whole way to AIs are so much better than we are, we are ants to them (unless Shep is supossed to shoot the robots in the head) and then the unstoppable juggernauts felt threatened by organics. Question is, how do you threaten the juggernaut and last time I checked, these guys weren´t feeling emotional. Ok perhaps I am simply not in the right mindset to see "it wants to become even more intelligent by harnessing the energies of the whole galaxy," I just wonder why bother? There is no point in it. It´s purpose was probably eliminated when it killed its creators. Now the ASI is a thing without any purpose at all and no self preservation instincts.