It's important to point out that it's low speed, aging infrastructure that's a big problem for French rail. Bullet trains make a lot of money and in general people complain because as a result the SNCF invests a lot more into the high-speed infrastructure than to maintain the old, slow, regional lines.
The high-speed axes like Paris<->Marseille or the Paris<->Bordeaux are definitely not underused and the tickets are quite expensive, often more so than plane tickets but trains are generally a lot more comfortable and convenient (and about as fast or even faster door-to-door).
More broadly the problem is whether you consider that the SNCF should be run uniquely for-profit (in which case they'd probably end up closing all the small regional lines and only run the bullet trains) or if it's a public service that can lose money if it provides an important service for the citizens (in which case it makes sense to maintain the local lines even if they lose a lot of money).
The high-speed axes like Paris<->Marseille or the Paris<->Bordeaux are definitely not underused and the tickets are quite expensive, often more so than plane tickets but trains are generally a lot more comfortable and convenient (and about as fast or even faster door-to-door).
More broadly the problem is whether you consider that the SNCF should be run uniquely for-profit (in which case they'd probably end up closing all the small regional lines and only run the bullet trains) or if it's a public service that can lose money if it provides an important service for the citizens (in which case it makes sense to maintain the local lines even if they lose a lot of money).