[They only exist for the disadvantaged. I could move to any country I choose.
Absolutely true. They exist to protect the privileged from outsiders. It is amazing how much easier it is for me to travel around the world as a Canadian citizen than it is for my cousins to do so as Filipino citizens. Astonishingly so. I can visit them without any word to any government body and stay for weeks. All I need to show is that I have an onward ticket for the end of my stay. For them to visit me here, they need an invitation letter, and a tourist visa that is likely to be declined if they can't show that they have $3,000 Canadian in their bank account, a family and property back home, etc.
That indicates that they are a fiction whether or not I "oppose" them.
A fiction? No. I'm afraid they are quite real. And they exist to protect your privilege.
I don't think the world would be significantly changed in their absence, aside from the necessity of coming up with a new reason for the proletariat to hate each other.
In their absence your country would collapse under the strain of too many people coming in, and coming in unvetted, until your country declined to match the countries the people are coming from. It would be incredibly noble of you to give up your privilege like that, and would help billions of people who now live in third world countries. Be aware that you may be far down the totem pole within your country, but you are at or near the top 1% globally.
I don't actually oppose the existence of national boundaries, but using them as an excuse for bigotry and a blank check for violating human rights is morally repugnant.
Absolutely. And I don't think anybody here would disagree with that.
Immigration is a positive, not a negative to western countries, and I would encourage more of it. But it is a positive becasue it is controlled. The vetting actually allows you to take the most productive of other countries to add to your economy. That won't be so if everyone is simply allowed in unvetted. And the reason undocumented immigrants tend not cause huge problems today is because they are illegal and have to keep their heads down as a result, so to not be found out and deported. Them being illegal also discourages them from coming in higher numbers. If you want to keep your lifestyle, your culture, and your relative global privilege, you do need controlled borders. If you are willing to give those things up so people can have a better life, I applaud your kindness.