The thing is: I'm not so sure airlines want to "share" those extra special customers that much.

Let me explain: Sapphire, Emerald, etc are frequent flying levels where similar benefits are extended by airlines which can therefore offer to give them to each other's elite members (lounge access, extra bag, etc).

By contrast, those "plus" levels are where you typically start seeing serious value as an airline and try to recognise it by more personalised/informal support, and paradoxically, trying to extend reciprocity presents a major risk for the airlines doing it less well: you may well end up having your best passengers realising that they might be better off with someone else. What if you find that with such status, airline OW1 gives you space available upgrades on their long haul flights when BA does not? (I know we all already know AA gives upgrades to their elite on domestics, but frankly it matters little as US domestics are not a core route for most BA GCH, GGLs, etc so we are not going to switch allegiance just for domestic upgrades when we fly AA Y 4 or 5 times a year. Now if it was transatlantic flights on the other hand...) What if you find that airline OW2 gives you personal escort service at the airport while on transit while your airline doesn't? What if you find that airline OW3 waives ticket change restrictions for its OWE+ members whilst your airline does not, etc. All those things could actually be more negative than anything else for your "Base" airline a it might well encourage some people to see that there is a big wild world out there. Of course, you can already see some airlines have better lounges or better priority luggage deliver than ours etc but I think that this is still a limited issue compared to entirely different treatments of super elites.