Trust me, you're not the only one who has said these things. I am fully aware most people disagree with me, haha. I've had some explosive arguments in real life about this, because ultimately it's a very.. emotional subject for people.
Just to respond to your points. I think "tradition" is never a good argument. Slavery used to be a 'tradition', stoning people used to be (and still is in some places) a tradition, circumcision is a tradition. None of these are good just by virtue of being a tradition. Traditions can and should change, especially when they're based on ignorance or hatred or actively harming people.
I am not religious and my example about the soul was really just an example. I know plenty of self-proclaimed christians who use their religion as a reason not to be a donor, and I just don't understand it.
Why should the relatives of the deceased have the final say on it? Is the corpse their property? If it were, they could choose to take it home and let it rot in the basement. Or cut it into pieces and feed it to dogs. Or use it for target practice, stuff it like a piñata. I wouldn't be surprised if there are places in the world where people are allowed to do that, but where I live it's not allowed. So if the corpse is not their property, then whose is it? Not the deceased, because they're dead, they don't own anything anymore. Not the relatives. Then who? If nobody is the owner of the corpse, then why shouldn't the state use the parts that can help people?
Look, I know there is no country in the world that will ever implement what I would want, but at the very least they should all make it opt-out, because this way people who don't care (and wouldn't take the time/effort to register as a donor, even though they wouldn't necessarily mind being one) will automatically be a donor, and the people who are really against it will opt out. Also, if they refuse to make it non-optional, they should make it so that non-donors cannot receive a donor organ themselves, or automatically move down the list when a donor needs one.