I know nothing of france's legal procedures, but it seems strange that the lower court, after being ordered something, could drag its feet for so long.
It doesn't provide any feedback on the actual judgement it overturns (or not). So the court that does the actual judging is free to get to the same judgement as before if it respects the process correctly this time. But this is essentially a new judgement that starts from zero and takes as much time as the first time.
Did the court de cassation overturn every single detail pertaining to the original judgement? Kinda feels like they should be able to speed up the new judgement, and only change whatever detail or details the superior court said were wrong.
Well, I don't know much about this subject, but these delays make justice less just, and I don't understand how this is not obvious to everyone but the guilty.
I believe what contributes most to the delay though is just that the French legal system (like most others) is overloaded and it just takes time until the case gets to the front of the queue.
The cour de cassassion can't rule on the case. It rules on whether the previous ruling was made properly according to legal standards. It is possible that the cc broke the previous decision for a reason unrelated to the case matter, and so the lower court can reissue a decision respecting the spirit of their first decision, but without the legal error.
It's not very unusual for criminal cases to drag on for a decade between arrest and end of all procedures, for instance.