His whole concept of society and the economy is somewhat outdated.  Jaures sees a society of rentiers and owner-capitalists vs. everyone else.  However, even when he when he was writing the modern professional and managerial classes were beginning to emerge, and today are at least as important as the other two wealthy groups.

But it feels like the left has given up on changing society. Is this idea done for?

I certainly hope so, at least in the terms we're talking about here. Every attempt to completely eliminate the private economy has ended badly. On the other hand the social market model of postwar Western Europe created a very good society - not perfect, but quite good in spreading prosperity to most of the population.  Why would anybody in their right mind support something which has always failed when there's a good alternative that has worked. You might say that socialism has never been done the right way, perhaps. But how does that differ from the radical neo-liberals who argue that the full blown, completely unfettered market economy would bring prosperity and happiness to all, if only it were done the right way?

Incidentally, I'm not sure why you view Jaures' concept emphasis on the nation here as completely outdated. He is simply saying that given that the fundamental unit of political organization is the nation state, one has to work within that structure. Present day Europe is different due to the EU, and international organizations like the WTO and the IMF play an important role in imposing and regulating socio-economic systems, but even now the individual nation-states remain very important, particularly outside the EU. Jaures' emphasis on the nation is thus only partially anachronistic, just as in the case of his view of the upper class as made up of owner-capitalists and rentiers.