Maybe he preferred a purely happy ending where Tony and Pepper lived happily ever after and he got to watch Morgan grow up.
This is purely conjecture, but maybe Robert Downey Jr. liked that Tony Stark got a poetic and heroic end, but had hoped that he wouldn’t die at all.
So he may have accepted and appreciated the ending on a story level, but still wished that this character that he played for so long and was paid so handsomely for could have survived.