Just so. It was a mystery, and it is a mystery no longer. Which thus invalidates the argument that the Romans had materials that are not available to use, and that therefore we can't build durable infrastructure like theirs. At the present time, we
can do that.
My argument here is that in many (though not all) cases we
should do that.
Because it is more expensive initially, but saves on repeated replacement costs later.
Certainly, after over 2000 years,
nothing is going to be in perfect pristine condition. The point is that modern roads are designed(!) to last 50 years at most, and often last only a few years before developing major failures like potholes. Even in an over-organised country like the Netherlands (whose road system is regarded as one of the best and most orderly in the world), asphalt roads last less than a decade before they need major work. And I'm not talking about just the highways that take convoys of trucks all day. I'm talking about streets in sleepy suburbia, that get the least "inflicted" on them.
Now let's look at the "bumpy and difficult road to travel on" that you refer to:
As I mentioned above: it's not a suitable highway (even if you broadened it), and it wouldn't be even if new. But for a literally ancient road, it has held up better than anything we build today! This kind of magnificent, durable work would be excellent for local roads in many places. You can put it there, and be certain that it
stays there. You won't have to come back next spring and basically re-do the whole thing.
Hilariously, parts that have over time been repaved in a less "rigid" manner (i.e. not in keeping with the Roman standards) have stood the test of time less impressively than the parts that were maintained in the Roman manner.
Some further examples:
We are looking here at roads from Antiquity. These things hold up. And if you look at Mediaeval paved roads, these are less sturdy, but those also hold up better than most paved roads built in modern times. (Modern paved roads tend to suffer from sinking stones, because the foundation is less well-made, while asphalt is just inherently more vulnerable to deterioration.)
For some further evaluation-- below is an image of a Roman bridge that was used for traffic into the '90s:
Show me a modern bridge that you truly, honestly expect to still stand there, 2000 years from now. Show me that bridge. No such bridge exists. We don't make 'em like that anymore. We really don't. (And ironically, the examples you might point to that will last
longest, of what we have now, will also be the
oldest ones. From when we still made them out of stone.)
Modern infrastructural construction is
crap. We should do better, and we can do better. We can learn from the Romans. We can build for eternity. Not every house, not every shed. But the big things. The ones that
should last.
It's not impossible. And that's my point.