No, it's not. Every other form of transport exists to get you somewhere, space travel exists purely for the traveling, it has no practical application. If there were cities in Europa or something, sure, you'd need to use a spaceship to get there, but there isn't.
The whole 'pushing the boundaries' thing only applies when there is a tangible result. Establishing the US is a meaningful, impactful goal, so lives lost towards that goal have a sacrificial value. Leaving the atmosphere for the sake of doing it isn't worth even a single life, let alone how many it's taken, and will continue to.
And yes, everyone will have their own value of human life. Some people would choose not to wear a helmet when riding a motorbike too, but if it was my decision, manned space travel would be over, long over actually.