Unfortunately the Warranty is as much for the manufacturers benefit as well as the customers and it totally limits their liability, even if it turned out there was a defect in the frame when originally sold. If they’ve done something for you outside of warranty its ex-gratia (a defined legal term) and is not an admission of anything – simply a good will gesture at their discretion, nothing more, nothing less. I’m afraid you would have got nowhere if you had tried to push harder especially if you started accusing it of being not fit for purpose. How would you ever begin to prove that? Certainly not from the opinion of a frame builder, no matter how well respected.
Could you imagine if everyone with a car that is out of warranty that has broken down approached the manufacturer demanding a new car, or free repair, claiming the car was not fit for purpose? You wouldn’t get very far.