France is getting more zones where you need the Crit’air stickers. Despite some people thinking French people don’t bother with the stickers, they are quite commonly seen on vehicles now, especially close to these zones and if you drive into a zone without the correct sticker, you will get a fine.
The stickers last the life of your vehicle, so if you buy a Euro 5 or 6 diesel, you will get a class 2 sticker which will allow you to drive right into ( God forbid) Paris among other cities.
Of course, things may change in the future, access by classification might change, but permanent emission zones are in the big cities, and I for one would rarely visit a big city.