While it's true that rear derailleurs have evolved significantly, I wouldn't necessarily attribute this progress to "drivetrain perfection." Derailleur development has indeed improved shift performance and precision, but perfecting the drivetrain is a complex matter involving multiple components, not solely the rear derailleur.
In the past few decades, advancements such as wide-range cassettes, Shadow technology, and multi-shift actuation have improved shifting and reduced chain slap, enhancing overall performance. However, this doesn't mean modern bikes always boast superior drivetrains compared to their predecessors.
The notion of "perfection" can be misleading, as it often ignores factors such as maintenance requirements, compatibility challenges, and the learning curve associated with new technology. For instance, integrating electronic shifting into a drivetrain can be expensive, finicky, and demand specialized knowledge, making it far from "perfect" for some cyclists.
In conclusion, rear derailleur advancements have played a crucial role in enhancing cycling experiences, but it's essential to maintain a realistic perspective when evaluating their impact on overall drivetrain performance and reliability.