I'm just pointing out that when on another world, at least days from help from home (maybe months, if they have to put together a totally unscheduled relief delivery), you cannot expect a solution that might be routine on earth to suddenly materialize on your doorstep. Not even Amazon can do that.

And that spare part might mean stopping the project altogether until it shows up, or the death of all the astronauts. Just because something can be done easily and quickly here on earth doesn't mean it can happen overnight on mankind's most remote outpost. Every emergency that can possibly happen cannot be reliably anticipated. The definition of an accident is something you were not ready for, no matter how well you planned and prepared ahead of time.

I've lived on a ship at sea, an environment where you are totally surrounded by a hostile nature that might kill you in minutes. I've also owned sailboats and spent a lot of time trying to think ahead to every possible crisis and how to best respond to it. We've had centuries to work out the tech and procedures and training and redundancy to do that safely, but ships occasionally still have to limp home crippled, or get towed to a safe port, or disappear and are never heard from again. Yes, there is a work-around or a solution to ANY emergency, a spare part or backup that can save your ass. But it is impossible to carry a fix to EVERY emergency, especially the ones you can't anticipate. And every single one will have to be dragged up the gravity well at great expense.

The spacefaring nations are very well aware of his, and they've done a great job of keeping their crews safe, and even pulling off virtuoso rescues like Apollo 13. But people have died in space. And they will continue to die. More and better tech is not the solution to every problem. Sometimes you just have to think things through; not only must you be prepared for emergencies, you have to design your missions so emergencies are less likely. And sometimes tech just gets in the way, or gives you a false sense of security.

Case study:
I live in South Florida. The area is heavily criss-crossed by canals, dotted with lakes, and you are constantly crossing bridges or driving by bodies of water. Every night on the news you hear about people who have driven into
deep water and drowned because they were trapped in their cars. The water pressure won't allow you to open a door to swim out, so their cars slowly fill up and the people die strapped to their seats. In modern cars, you can't even roll the window down to equalize the pressure because there are no mechanical window cranks on modern cars, the windows are electrically operated and short out when submerged. The tech apologists say; "No problem, you can buy a ball-peen hammer and crack any window, the water pressure will break the safety glass into nuggets, the car will fill up and you can open the door and swim out."

Sure, after a violent crash, you're going to be able to FIND that hammer, no matter where in the car it is now or how disoriented you may be. You first instinct and muscle memory is to turn that window crank but its no longer there. You're hurt, panicking, maybe upside down and strapped to your seat, tangled up in a deployed air bag. Your passengers are screaming the awful shrieks of those who are in great pain and afraid they're about to die. You've probably never heard that terrible sound in your life and nothing's prepared you for it. And the car is quickly filling up with water, in total darkness.

OK, ball-peen hammers work just fine. They are great technology and can do the job perfectly. Its all you need. Right.