> It seems the network of roads built in the 40s, 50s and 60s just can no longer be done efficiently.
Of course they can, we have not lost that capability. It's not a matter of efficiency but effectivity. Road constructions of the 1960s are not effective for 21st century traffic demands. Today's level of traffic far exceeds the anticipated level of traffic at the time of construction. Germany sees this all the time, esp. with regards to bridges. Maintaining a road or a bridge to be effective at supporting the original traffic levels is easy but under today's load would require constant maintenance to not deteriorate immediately. Those constructions need to be upgraded which is hard to impossible to do in situ.
Let's take a well-known construction in Germany, the Leverkusener Brücke an der A1. It was originally built in the 1960s for a traffic level of 40,000 cars (and trucks) per day. It was upgraded and refurbished over the decades (meaning almost constant construction work happening) to a level of 100,000 cars per day. It wasn't enough, in 2016 about 120,000 cars crossed the bridge per day. At the same time trucks got about 30 % heavier from 1960 to 1990 and we all know that passenger cars got heavier, too.
So the whole bridge was replaced, which took 7 years, ending in 2024. During that time traffic was rerouted over two nearby bridges in Cologne and Düsseldorf. The Cologne bridge was so badly damaged by the additional load that it had to be partially closed down and now is up for refurbishment or, maybe, replacement. Network effects at work ;)
Anyway, what I'm trying to say is: we are actually better at building stuff than our predecessors but the demands put on our constructions are much, much higher. I don't dare to say if our capabilities have grown as much as the demands require.