It's like the saying, "It's always in the last place you look". The reason they stop developing antiquated technology is because it has become outdated. And because people stop investing in improvements, the last of the generation is therefore the best of it.
Take the XC-99. It's beautiful, to be sure, but is it the best possible piston powered plane? Could better materials provide a different design, improved speed, more internal space and a higher carrying load? Could better engines provide a better gas mileage, higher power output, less noise, better performance at altitude? I think you can see that improvements could be made. They weren't because it didn't make sense to.
To say that the technology is best when it's ripe for replacement could just be flipped around. Technological advances happen when they happen and whatever gets replaced was the best we could do before then.