Re: "it was required to keep a second shuttle on standby just so they could use it as a lifeboat"
"And I find very funny that in the late 1970s they could handle Shuttle complexity, but in the 2000s they could no longer - nor devise ways to reduce it with new technology."
That's not really true. They couldn't properly handle the shuttles complexity in the 70s either. Just look at how the Challenger disaster happened. And they COULD handle it in the 2000s. The problem with the shuttles wasn't that they're too complex. It's that they are too complex for what they do. NASA could handle it, but after the ISS was completed that level of complexity just wasn't worth it.
The fact that there was no way to recover from SRB failure was a massive issue. The fact that there was no proper way to inspect or repair the heat shield on orbit (until they cludged something together after Columbia came down in flames) was a massive issue. The fact that it AT BEST had a 30 day on orbit lifetime was a massive issue. The fact that the heat shield was easily damaged on launch was a massive issue. Etc. STS has MANY shortcomings
The fact that there was no replacement after the shuttles were retired is down to politics and mismanagement, which is the real issue here. the US should never have put all its faith in JUST the space shuttle. If it had kept a separate man rated heavy lift capability the shuttle would probably have been retired sooner and probably not nearly missed as much.
Don't get me wrong, I LIKE the shuttles. They're really cool technology. They just didn't live up to their potential and had many many shortcomings that were never addressed for one reason or another.