Decency, like all moral words, has a fundamentally subjective and socially constructed meaning. Decency is we think it is, and what we think it is is a product of social interaction. But just because its meaning is subjective and socially constructed doesn't mean it has no meaning. It does mean something: although we might disagree about the specifics, we can tell what is decent and what is not decent.
I think, for example, that people have to live with no home is not decent. That they have to beg for food is not decent. That they are not permitted to work — when there is manifestly much work undone — is not decent. That we are killing black people in our own country and brown people in the Middle East is not decent.
Others might think, for example, that we take from those who produce more and give to those who produce less is not decent. I disagree,
No matter: the point is not what specifically is or is not decent, the point is that the word decency does actual work in drawing distinctions about the world.
Decency is more fluid than good. I've written earlier that "I shouldn't, but ..." is incoherent. Shouldn't means don't. If someone actually does something, then they necessarily think they should do it. If they say they shouldn't but they do, then they are lying, bullshitting, confused, or so neurotic that they need the services of a psychologist, not a philosopher. Decency, in contrast, is not so rigid. Should and shouldn't come after we weigh the reasons; considerations of decency come before.
We can say, "This is not decent, but reasons." And the reasons might (or might not) be good reasons. It was certainly indecent to kill Nazis by the millions, because mass murder is, I think, uncontroversially indecent, but hey, they were Nazis. (And, I think, the analysis is symmetric: I think the Nazis and Germans believed that murdering millions of Jews, Gypsies, communists, homosexuals, and Slavs was indecent, but hey, reasons.) Similarly, whether or not someone thinks the economic constraints justify the indecency, it seems relatively uncontroversial that the way we treat food animals is clearly indecent.
The goal of civilization, I think, is or should be that we create a society where we can not just always act rightly but always act decently. A goal of universal decency might be asymptotic, but we should always at least be moving closer, to make our necessary indecency always rarer and always more fraught.
What I meant in my previous post, then is not to argue some specific concept of decency, but to talk about an attitude towards how we construct and implement not only our notions of decency but also when we make exceptions to decency. Hence, even when I completely agree with some religious people's specific constructions of decency, I profoundly disagree with how they construct that notion: that thus and such is decent or indecent not because we happen to subjectively feel it is so, but because God has so informed. Similarly with a monarchy, oligarchy, or even a republic: even if I agree, thus and such is decent (or we should make an exception to decency) because the king, or the bourgeoisie, or our elected representatives have so informed us... based, of course, on information only they can see.
As I noted, the specific institutions of a democracy are important, but democracy is more than just a set of institutions: indeed, no set of institutions, however carefully crafted, can be democratic if the people do not have democratic consciousness: the consciousness that the people themselves decide what is decent and when we must act indecently. Not the king, not the elite, not the trustees (and not the Party): the people themselves.