It's really hard to say. I'm guessing that for a Marine who wants to leave after witnessing the event, it would be about timing as well as their reason for resigning. If their resigning even seems a bit suspicious, Cipher Pol would likely be involved, or perhaps even admirals to hunt them down.
I'm not sure if Garp had ever attempted to resign (besides after Ace's death), but I'm willing to guess that Garp was hoping to play the long game with the Marines. He's well aware of the corruption within the World Government, and possibly believes that he alone wouldn't be able to change it. And he doesn't seem to share Dragon's views of using force to change the World Government as he still believes in the potential it represents for the people. So Garp bets on the future generation to achieve the change that he couldn't, hence his willingness to save Koby as well as his hopes for Luffy and Ace to become Marines.
Garp probably thought that by raising the new generation properly, that they would go on to raise the next generation properly as well, and so on and so forth until the World Government made up of people the right idea of justice and can bring a sweeping and peaceful change through it's inner working. The only issue with this idea is that the World Government is run by seemingly immortal evil beings, which I'm willing to believe that Garp himself is unaware of.