ranskis

The big mistake of these airlines is to have the light/classic/flex in all booking classes.
The "flex" is actually just a slightly discounted change fee paid once upfront that allows for several changes, so it makes sense to keep it for all booking classes.
However, how can you charge 2000 EUR for a Y light ticket and tell the customer at check in that a bag is 100 EUR extra, that he had to buy the classic fare to get a free bag? Or tell him that he cannot change his ticket at all at this price point?
In all logic, light fares should only be available on cheaper booking classes, the classic fares on most non-full fare booking classes and only flex for the full fares like Y B M fares: you don't nickel/dime customers on the most expensive tickets for bags and ticket changes.

Edit: I should have made my little research before posting Y B and M fare CDG BKK are only available in full flex. However they cannot be combined with the cheapest economy fares in the other direction, N class minumum it seems. Then Y and M fares can be combined with a light fare (and most likely classic as well) but the B fare requires a combination with a flex fare. There are still some consistency issues!

irishguy28

Just as they serve different meals, some have Premium Economy cabins, etc.

Seat fees are not mandatory. Unlike the case with checking a bag, every passenger will be provided with a seat without needing to pay. If a passenger wishes to pay in advance for a "better" service, that's entirely up to them, but not part of the "standard" service.