Focusing on your question...
My question is: do we observe that spaceflight costs are indeed reduced with technology improvement?
I studied this very topic for the Space Shuttle program and NASA's Commercial Resupply Services and included my findings in a paper published last year.
Keep in mind while reading this that what this article is discussing is the cost of sending cargo (and sometimes crew as well) to the ISS's orbit. These vehicles are designed to also safely reenter with cargo (and sometimes crew) so they can't deliver as much payload to orbit relative to a system that only launches satellites. This leads to higher apparent per-kg costs.
To arrive at a cost-versus-year trend for the Space Shuttle I assumed that all Space Shuttle missions were ISS missions, and then plotted the total amount of payload delivered versus the amount of money that was spent on the program, using 2023 inflation-adjusted dollars.
This approach gives us the plot shown above in Figure 3. By fitting a curve to the data in Figure 3 and plotting the inverse slope of that curve versus time, we end up with the orange curve of a USD-per-kg versus year shown in
We can see that the Space Shuttle’s cost-per-kg came down over time, but we can also see that there are a few peaks and valleys in the curve. The peaks were caused by the Challenger and Columbia disasters. Between these
events, cost dipped to as low as 59,000 USD/kg.
If we apply the same methodology to Commercial ISS resupply services in general (blue curve) and the cost with the market leader, SpaceX (purple curve), using data from usaspending.gov, we observe that in practice the costs have only recently managed to achieve cost parity with the Space Shuttle Program. An Aug 2018 independent audit from the Office of Inspector General, on page 27, projected that the cost of commercial ISS resupply with
SpaceX would be 71,800 USD/kg, which aligns well with the usaspending.gov data shown in Figure 2.
Other corroborating data includes an Aug 31st, 2022 press release by NASA where they announced that they had awarded five additional missions to SpaceX at a cost of $\$$1.436 billion or $\$$287 million per mission. This places
the cost of future resupply missions 10 through 14 at 1,436,438,446 USD, which works out to 86,794 USD/kg if we assume that each mission delivers the maximum payload of 3307kg to the ISS. On March 8th, 2023, Robyn Gatens, Director of the International Space Station at NASA, stated informally during a Q&A session at the IEEE Aerospace Conference that half of the ISS budget
goes to launch costs. As NASA spends roughly 3B per year on the ISS, and resupply runs deliver people and cargo at a rate of roughly 20,000kg per year, this works out to...
$$3,000,000,000 USD / 2 / 20,000kg = 75,000 USD/kg$$
The paper also traced the source of the Falcon 9 information on the Visual Capitalist chart (published 2 years ago on January 27, 2022) to an estimate made back in 2016 by The Tauri Group. If you're interested in those details, you can find the full paper by searching for it on Google Scholar. But the Falcon 9 information was also for launching satellites, not resupplying the ISS.
Regarding the Falcon Heavy data point, the article makes the following observation...
If Falcon Heavy were cheaper
than Falcon9 as the chart suggests, then one would expect
SpaceX to use Falcon Heavy to launch its Starlink
satellites. In practice, the Falcon Heavy may be less
reusable than the Falcon9 because the Falcon Heavy has
not yet demonstrated the ability to land its core stage
downrange on a drone ship. SpaceX’s actions may reflect
an internally held belief that using Falcon Heavy would
be more expensive than using Falcon 9 for launching
So far as the US government and its taxpayers are concerned, we are not observing that spaceflight costs are trending down.
Of course, the Space Shuttle is getting creamed by SpaceX's rockets in the court of public opinion. However, a more considered engineering analysis shows that if you consider the class of missions that involve sending a vehicle to a LEO rendezvous and having that vehicle return to Earth safely, the Shuttle was a cost-effective launch system.