https://www.nightjet.com/dam/jcr:6a8041cb-0131-4ad3-84fd-251...
The biggest issue is that they tend to be booked up weeks in advance so getting a slot in private compartment is pretty hard.
That’s France’s rail network in a nutshell. It’s great, if your trip starts or ends in Paris. If you’re not, you might still go through Paris and driving could be faster than a high-speed train.
The other issue with domestic sleeper networks in France, other than sounding like a terrorist, is that high speed trains usually take 3-4h anyway.
I think the only takers for an overnight equivalent would be tourists that can save a night of stay somewhere. And that’s if you’re not staying with family or friends.
And the trains are almost all electric, so it’s harder to make an argument for “slower, therefore less carbon burn”.
It only seems useful if you're Paris based, since this appears to be the center node of the network.
edit: This at least appears to be the case for the French startup mentioned and the map displayed at the very top of the article.