I also wanted to say: Libertarianism ia a journey, not a destination. It's been described as a bus going towards some goal of government cut to the bone, and you and I may choose to get off the bus along the way. If government in its current state is a bus, a libertarian might want to get on to slam on the brakes, turn the bus around, and get it going in the right direction. If you're just continuing on the same route, but slower, that's a good indication that you've lost the principles you started with.
You correctly point out that the people who govern us are not machines or angels but just regular people who often lose their way. People who are under tremendous pressure to compromise their principles. When we give more power to government, we are giving power to these people. In my mind, there's not a lot of difference between a billionaire business mogul calling the shots for his own benefit, or a millionaire politician calling the shots for his or her own benefit. It's still all about power. It's only the message that changes.