If France wants to win the world cup then they have to avoid NZ until the final (assuming NZ reaches the final). I don't think any team can knock out NZ prior to the final and go ahead to win the RWC. Only happened once in history (1991). For many teams the effort and elation from knocking out NZ is equivalent to winning the RWC. Hence mentally and physically not being able to repeat the performance again in the next knock out games.
France they are the perennial chokers at rugby world cups. Maybe chokers is not the correct term as it is reserved for NZ however they have probably been in as many Finals/Semi finals as NZ but not gone on to win.
The best chance to win the RWC will be to play NZ in the final as France know the history, if they beat NZ in the QF, chances to go on and win the world cup will be slim. So I think it all depends on French mentality, whats more important to them, winning the world cup or beating All Blacks. That mentality will be shown in their final pool match against Ireland - that performance will indicate whether they reserving themselves for NZ the following week.