You don’t seem to have grasped my point or what the OP’s actual situation is and I have no idea who exactly you are arguing against.

BA has a legal obligation to rebook under comparable conditions and in the same class - indeed it doesn’t matter if EK F and BA F are the same standard or not and ‘F is F’. However, the OP’s request here to move from a BA F ticket with a connection to a direct EK F ticket is likely outside the scope of this obligation as has been noted above (this is different than rebooking a LHR-DXB F ticket onto a direct EK LHR-DXB if BA can’t offer a same day BA direct alternative). Nevertheless, my point above is regardless of the nature of the legal obligation, BA is taking a commercial decision here not to offer rebooking and the reason BA won’t do it in practice is they have no rebooking agreement with EK. Therefore it very much does matter here that EK F is better than BA F and considerably more expensive, since this makes it even less likely BA will rebook onto EK and take a massive loss on that ticket as a result (maybe they will for a very valuable customer like a Prem as has been noted above, but someone else has no chance).