I think the French bishops are worrying too much about this. The permission to say the 1962 Mass would not be a mandate. This indult, which has not yet been issued and might not be, is not going to cause as much change as some think. It would be up to individual priests as to whether or not they want to celebrate the Tridentine rite and the fact of the matter is that most will choose not to do so.
I think the whole thing is symbolic. It will probably have little effect on the practice of the French bishops, who will probably politely ignore it. They simply don't like the idea of any approval being given to it, because they fear it might weaken their position. Since their position has led to a church in which something like only 3% of French Catholics attend Mass anymore, one would think it an undefensible position. But not to them, apparently.