I maybe wrong in this but I thought that the codeshare airlines will get a percentage of the ticket. So if say the ticket is FLR-FCO-NYC-AUH-DUS for $1000 then every operating airline will get a percentage according the distance flown on their metal. So NYC-AUH would get the major chunk followed by FCO-NYC.

This would explain why LX and AF did not let people fly on the RGN fares even though SQ or others had brought them to Europe as part of the tickets.

If they were going to get the full fare then there is no reason why they would say no you can't fly as ticketed. LX and AF downgraded pax to Y. They were not the issuing carrier so would not have been responsible for paying up.

So in this regards no ticket is safe. We were in Asia 6 months after booking the ex RGN fare and had our tickets cancelled 3 days before departure.

Always have a back up plan and don't rely on only this as your ticket home. These are not tickets that you book for your grandparents or novice travellers. If they work out then great just don't plan your honeymoon around them.