I won’t disagree with anything you say save for the last sentence – they DID lie to me – they specifically said that they could not check me through “because we don’t have a baggage agreement with Thai” (exact words used). And that’s just not true. TG’s own website says they have one, and in any case they must have had one because TG interlined onto BA (on the same tickets) on my outward flights.
BA *could* have said that as a matter of policy they don’t interline to non-alliance customers on separate tickets – which would have been both fair and accurate, and which I would still have found disappointing but would have accepted. Instead, however, they tried to present it as an impossibility rather than what is presumably a cost-saving measure.
I’m old-fashioned. I don’t mind being told I was trying to pull a fast one and I am not getting away with it. I *do* mind when someone to whom I am paying a considerable sum of money declines to accommodate my request (even one to which I am not strictly entitled) but blatantly lies about why. It is also a particularly stupid approach to take with a frequent flier who was able, in less than a minute, to prove that what they said was a lie. That doesn’t “soften the reality” at all – it turns me from someone who is disappointed into someone who is disappointed and angry.