In France the track usage fees are very high, and you need state support to be able to cover them. Germany, for example, doesn't give such subventions to night train companies.
European Sleeper can run to Paris because they have chosen a route where they minimize the kilometres run on French territory.
But indeed: Where are those lines really fighting closure? Currently all of Europe's night train wagons are in use, and if some connection is closed, the same amount of capacity (the same wagons) will appear elsewhere in Europe.
I don't think we'd be having trouble with the availability of wagons if things were as you claim.