> From a taxation point of view I'm under the impression that for a country such as France for example seniors are gonna get _on average_ way more benefits during their pension than the equivalent of their lifetime contribution. Mainly due to demographics and life expectancy evolution.
(I'm making a digression here from the main topic, so don't read on if you're not interested.) I'm not sure why you would think pensioners in France will get far back far more than they paid in.
Under the current system in France, when you're working, you pay into one or more of the dozens of different pension plans. Each plan is designed to be self-sufficient. It receives money from those who are currently working, invests or saves the money, and pays out those who paid in in the past and who are now retired.
Doing things this way has several pitfalls. The main problem is that if demographics change (greater percentage of pensioners as compared to workers) it puts a strain on those left in the workplace. This doesn't mean however that each pensioner is getting more than they paid in, it just means that proportionately there are less people left paying into the plan.
The retirement system is currently being revised. In the future, there will be one universal plan where each euro paid in will buy a certain number of points, which will have some value when you retire. Since the value of a point can fluctuate, pensioners may end up getting more or less than they paid in. (They'll get what the plan can afford to pay out.)