Ligue 1 is still a decent level to amass that experience - with a huge chunk of that being the leader of his team.
i don't think his experience is necessarily the biggest issue. For his age he has great experience, he also seems like an influential character in the dressing room at Nice. I think the issue is that he hasn't necessarily kicked on since he first broke through and he still makes mistakes.
If you want to play a back 3 long-term then he starts to make more sense. But if you want to move to a back 4 then I wouldn't say he's good enough yet to do that for us, he should go to Germany and improve more over there.