Well, don't forget that - particularly when different airlines/ticket issuers are in the mix - there is also the question of who gets billed for the lounge visit.

I'm not sure AF will receive much payment if they started allowing access to their lounges for pax bearing arriving boarding passes operated by partner/alliance airlines, travelling onwards on separate AF economy tickets, and billing these partners for such access.

So while they may occasionally turn a blind eye to an arrival boarding pass that was flown on AF, they almost certainly will not grant access to everyone arriving on a Skyteam business class flight, who then travels onwards on an unrelated AF economy ticket.