The odd thing to me about France falling to this level of Poverty falls into a couple of categories.
1) With World War I levels of technology, having a significant (>40%) amount of the country destroyed in the course of the War isn't likely, there are chokepoints on the borders with both Germany *and* Italy. The Franco Italian border is probably *easier* to run barbed wire and trenches from End to End than the German border and the French will have an active ally in the Belgians, which more than balances out Germany having Luxembourg. (Belgium on the other hand will end up as bad or worse that OTL WWI)
2) Certain significant French Cities just won't be in the line of fire because it will be more worthwhile to make sure Paris falls first. Why does Nantes (to pick an inland city in the west of France *ever* get attacked? This is sort of the contrapositive to Germany in WWII against the Western Allies. To get to Berlin, you have to steamroll through 80%+ of Germany and Germany still doesn't have Berlin at its heart the way that France has Paris at its heart. (The question as to whether Germany packs it in if the Russian Empire conquers Berlin in WWI is an interesting question)
3) Let's say postwar that Germany and Italy in Europe take every square mile of land that was ever part of the HRE plus Corsica, and everything outside Europe except Algeria. (Not sure why they'd keep Algeria and not Corsica, but let's go with that.) They would *still* be one of the half dozen most populated countries in Europe, still be relatively prosperous and have an academic tradition that would keep them "First World". Sure, they could be put into a place where they'd be inferior to Germany for the rest of time, but they don't have the issues the Confederates do with 1/3 of their population (Negros) in an active revolt as long as a neighbor wants to.
4) 15 years after the war, I still expect them to be in the top 10 in Europe. They'd be behind Germany, Russia, Italy, the Ottomans (presuming the Italian/Ottoman war hasn't kicked off), Hungary(?) and the UK. Who else would they be behind? Spain? Bulgaria? Sweden? Greece?