What would get BA creaming in their pants now (and has been communicated to us) is the prospect of mixed fleeting the 777-9 with the 787 and doing something similar with the A350 and A380. I think it’s always going to be beyond them, however.
I think that was the general idea 10-15 years ago: eventually two LH fleets, one Airbus, one Boeing utilising the common type ratings. Lots of flexibility with inherent savings. BA have four now, which I suppose is better than the 757 / 767, Tristar, 747, 747-400, Concorde and DC-10 which probably all overlapped at some point.
It is true that pretty much all of the rumours about new aircraft purchases and fleet combinations are proven incorrect at some point - I don’t think anyone posting here has access to that level of information, and even if they did, they certainly wouldn’t be passing it on.
Would be interesting to see the fate of the 777-300s in this instance given that’s going to end up another small fleet. A top up order for A350-1000s could be in the offing for that if strategically the decision was to replace them but, as you said that won’t be for a while yet.
Looking at it logically (yeah, airlines...), without any insider knowledge, it would be more sensible for BA to add the 777-9 to the 777-300 as the -200 is decommissioned - they are very much alike in terms of systems, capacity, size, etc. and type specific training would be minimal for those current on the 777. There would also be ~34 aircraft in the combined fleet which gives plenty of options, like the 787 with 37 hulls now. Combine the 350 with the 380 and you get 30 airframes. Three fleets to the current four, roughly the same size, makes sense? Probably a non-starter for that reason, and that the 777-9 seems to be slipping continually in terms of delivery dates...