Of course you can live with only one functioning kidney, but part of what's so great about nature is that you have a lot of built in spares. One eye gets damaged? You can still see with the other one. Accidentally chop your hand off? Hey, look, you can still pick things up with the other one.
But, if you only have one of something, you do not have a spare. If you are in a car accident and your kidney is damaged, your SOL and now you are on the transplant list.
Personally, I have qualms about other forms of donation and am not ok with renting out peoples' bodies or paying for their body parts, but at least sperm regenerates and you'll never use all the eggs in your ovaries. Being a surrogate or harvesting eggs does involve risk, too, that's true. But why should we bring money into it, which may make people more likely to take risks they are otherwise not comfortable with?
If it was my mom or my kidney, she's getting my spare kidney just like she'd get my spare eye or hand. But I don't agree with enticing people with money. It's not about altruism, it's about not paying people to undertake risks that they would otherwise avoid.