Your first two arguments are pretty much indisputable if they're possible, and I really like #2 now that I know what it means (derp), but I don't see how #3 makes anything easier. It only means there will be a collective of donors on a server that will continue to grow. Sure, it's a better alternative to not kicking anyone, but as a whole, it's just...I don't know, redundant? Pointless? Why even kick players at all if you're doing it this way? Why not just kick them all? The servers are already absolutely overrun by people who have only donated so that they can kick people.
I can honestly say that I just do not have free money to spend on becoming a donor, and I don't want the server to be the sort where you essentially have to pay to play. If I ever do end up donating, it will be diamond, but right now, that just doesn't seem realistic.
I am being extremely selfish in my reasoning, and possibly a bit harsh on iron donors, and I understand that, but literally one afk donor can destroy the game for everyone. I am 100% against anything that makes it easier on iron donors. Which brings me to my replacement suggestion.
What if this perk is only available to staff and gold+ donors? There are a lot less diamond and gold donors than there are iron, and you ensure the people that actually care about the server get their guaranteed slots. This will result in less clogging, and extra perks for those players who go the extra mile on their donations.