It's still kind of surprising that the optimal design / speed for fuel efficiency has plateaued though, given how much more powerful the modelling tools are now

It's still kind of surprising that the optimal design / speed for fuel efficiency has plateaued though, given how much more powerful the modelling tools are now. The FAA certification overhead presumably has something to do with this. I was very struck recently looking at the Wikipedia pages for the KC-135 Stratotanker (first flight 1956) and its ongoing replacement the KC-46 Pegasus (first flight 2015). Just from the pictures of the two planes, I'd have no idea that one was more modern than the other.