actually there is nothing to be surprised about about this because football is a safe business, there is a huge turnover of money in it, Nantes is taking wise steps before their team falls into the brink of relegation, at least they must be able to survive in their current position. Currently Nantes' points are only a slight difference from those in the relegation zone, if they enter League 2 it will be very difficult to return to League 1. Hopefully Kombouare can improve the performance of the Nantes squad until this season is over.
Nantes must at least be in the top fifteen positions at the end of the season in order to secure itself in League 1 for next season, because it is true as you said that if Nantes cannot secure itself to return to playing in League 1 next season by reaching the same position. It's better this season, Nantes will have a little difficulty returning to League 1 because in every season there is always more different competition so that might be a quite real obstacle for Nantes.
Nantes has a difficult challenge ahead of them, preventing relegation will need a lot of commitment and work. If they can't rise to the occasion, there can be severe repercussions, including dropping to a lower league. As you have correctly noted, it would be extremely difficult to get back to League 1 after such a loss, and League 2's competitive environment would present further challenges. Nantes can struggle to make an impression at the top levels of league play if they don't make a strong commitment to getting better.
So, at this point, Nantes would be wise to do everything in their power to keep themselves from being relegated. Even though they have a long way to go, there is still some hope because the teams ahead of them are only separated by small margins of one to three points. Nantes therefore still has a realistic possibility, should they are persistent and determined, of keeping their league position.