And now we go to the ISS as guests in Russia’s Soyuz.

That’s underselling it. Hadfield again:

So I started studying Russian in 1993, I studied it for 20 years, I was NASA’s director of operations in Russia, I learned to fly, I learned orbital mechanics in Russian, vehicle control theory, studied and became qualified to fly the Soyuz. Then I flew as the left-seater in a Soyuz co-piloting, and a reporter says, “So, what’s it like hitching a ride with the Russians?”

Not only does he speak Russian, he speaks Soyuz! And hitching doesn’t require years of preparation:
Hadfield Training in Russia, short 7-minute YouTube clip.

Fun fact, as a fighter pilot Hadfield once intercepted Soviet aircraft for NORAD.

I recall when Daniel Goldin, as Administrator of NASA in the late 1990s, spoke confidently about shuttle missions until 2020.

It could have happened, but the Columbia disaster was the beginning of the end. Other than building, maintaining and supplying the International Space Station there’s not much requirement for its versatility. Servicing the Hubble Space Telescope was also exceptional, but the last time a shuttle mission actually repaired/rescued a commercial satellite was STS-49 in May 1992. So it was neither cost-effective nor lived up to its original billing, as satellite deployments shifted to expendable vehicles after Challenger, further reducing those potential number of missions.

It’s easy to get overly sentimental about the past three decades. And while I empathise with all those within NASA and its contractors who lost their livelihoods as a consequence, human spaceflight has to be about more than endless laps around the Earth. Though it makes perfect sense to do so on board a space station. The shuttle could have continued to carry more Spacelabs if there was no ISS.

He goes on further saying:

Those in charge of NASA as Apollo was winding down may have been fearful that the entire US space programme was in jeopardy.

They went on the offensive, exaggerating and promising all things to all people. Yet, a flight rate of 50 per year seems so absurd I find it amazing the shuttle was ever approved. Considering the sizable cargo capacity of the orbiters, one might venture to guess that overall shuttles flew half empty. Indeed, few missions other than the Hubble Space Telescope Servicing mission completely filled the cargo bay.

And user BNorton answers how the 100 mission standard was created:

It was a simple matter of structural fatigue analysis. The loads on the spacecraft during launch and entry, and the structure design, essentially established the 100 flight limit. If an orbiter had made 100 cycles, I would guess that it probably would have been inspected and potentially re-certified for additional flights/cycles. The same is true of all aircraft: they are operated in the US for a fixed number of cycles (take off and landing). For example, many commercial domestic aircraft are “retired” after about 60,000 cycles.

All things considered. I still think Hadfield is right!