/sarcasm
That sounds like an easy solution and philosophically appealing in a libertarian sort of way.
However, it doesn't work. People can be put under enormous pressure to make the choice that satisfies others - their boss, their peers, their community, etc.; it's commonsense experience, there is a lot of research about that, and a lot of implemented practices to prevent it in safety situations. How many priests in the Catholic Church or employees at Penn State allowed child rape to continue - something which provided zero benefit - under pressure? Or remember the article posted here a few months ago about the sea captain who, apparently under pressure, sailed into a hurricane and killed his crew.
Also, people don't always make good decisions; they shouldn't die for it, we shouldn't exploit them, and we don't want a system that attracts and promotes people for making bad decisions.
So, your alternative to letting someone else make their own decision is you (or someone else) making a decision for them?
All of the examples you just listed cut against your argument as much as they cut for it.
Bosses, peers, communities, priests in catholic churches or penn state employees _all_ have a history of abusing power as much as anyone else.
So, since abuse of power exists, and people can make good and bad decisions, it seems reasonable to expect a certain amount of bad decisions.
it seems safer to let individuals make their own bad (and good) decisions, rather than centralizing decision-making in a group of people that we hope will make good ones, but we must expect that a centralized group will make a certain number of bad ones.
Enshrining bad actors in a position of power is likely to have much worse outcomes than letting individuals occasionally make bad decisions.
Astronauts are a bit different: Highly skilled, with relatively good knowledge of the risks (but they are not professional engineers, for the most part, and are not immersed in the engineering of their equipment), and sometimes the reward is much higher (e.g., being the first to walk on Mars).
But the same mechanisms apply of risk, organizations, and the human response to them apply. Think of it: The proposal is to risk their lives so we can save money on fuel. That's disgusting.
Arguably it'd be riskier to use non densified propellants for these missions as then they'd be using different procedures than the cargo missions they launch regularly and will have less experience with it and opportunities to find and fix any issues on a cargo mission before it affects a manned one.
Then why increase risk to human life if there's no benefit?
It's highly debatable if there is a higher risk to human life as well - unlike with rockets where you fuel up before the astronauts board, you don't have people approaching a fueled up rocket which could explode with no possibility of escape. Instead if there is a issue while fueling up, the astronauts are in a capsule with a escape system which should pull them safely away from the rocket if there is a problem.
We're beyond that. NASA recently (within the last couple years) concluded an experiment where an astronaut stayed an extended period in space so they could study this, by sending one of a pair of identical twin astronauts. [1]
This is a press release about the upcoming experiment: https://www.nasa.gov/ames/research/space-biosciences/mvp-spa...