Unspoken is that a horde of Nilbog's creations rampaging across New Jersey might be the reason it was never safe to try anything before.
"Is it strictly necessary to remove him? If he's safely ordered down and you're consigning the space to the creatures with or without him there..."
"Absolutely. Even if your orders could be completely airtight—and you don't take chances with someone who's as dangerous as an Endbringer attack—there are millions of people terrified of him. Nilbog free but claimed to be toothless doesn't change that, Nilbog captured does."
"I don't have all the creatures' names. I'm not sure if all of them are people, either, so I might not be able to contain them as a group - he wants me to come back with an apology and a gift, and I imagine if my gift is a large basket of fairy berries a lot of the creatures will wind up eating them, but it won't work for any that aren't people or any who don't partake. If I could envassal all the people in there I could tell them to stay put and then I wouldn't mind if you killed anything that left, but that's a serious 'if'."
You could use the berries, command them to keep the other creatures inside the city, and make sure to warn them of why. And then bring more berries later for however many iterations is necessary."
"Can one of those precogs check my odds of success? ...Also, unrelatedly, I like this force field generator very much and would like to keep it."
That generator was not designed to be long-lasting, but we can discuss something more permanent when not in the middle of dealing with the world's fifth-most powerful villain."
"I do need to go back to Fairyland to collect berries, which will take a while."
"If a delay is inevitable anyway, we can have our Tinkers give you a force field that should last for years instead of days in exchange for another gate."
"The former. Tinker tech does have a tendency to break unpredictably even when lying unused, though, so it will not last centuries either way."
"Not long, but I don't have a good way to carry as many as I'm thinking from my tree to the gate or from anywhere into the town."
"If it's likely to take more than you can carry, we can send someone to help transport them. Some of us are less vulnerable to hostile fairies than others."
"I don't really want anyone I live near to know that I'm regularly interacting with mortals at all, but as long as I go through first to make sure no one's loitering near my tree and they can carry everything quickly it's not that risky, I suppose."
"I am Alexandria."
"Hi, Alexandria, I'm Promise. I assume that you, like this helicopter, fly faster than me; how do you want to get us to Brockton Bay so I can go to my tree?"
"The fastest way would be for me to transport you. I can get us there in half a minute, but it may be uncomfortable for you. Given the significance of the mission, I would recommend not waiting for a plane."
If they want to be in a desperate hurry about this guy who seems pretty content to sit in his town with his gross critters and do nothing else, that's okay. "I can do a reasonably good windscreen with sorcery. ...Do you have to physically carry me...?"
"Yes. I can't grant you flight.
On the way back, we'll have something more normal prepared."
Promise rolls up her wings as small as they will go and curls up. "Okay, if you're in that much of a hurry."
As it turns out, when Alexandria said it would take half a minute she meant because she is unwilling to break the sound barrier. The limiting factor is likely to be how good Promise's shield is.
Promise's shield is not good enough for breaking the sound barrier. It'll hold up with nothing worse than some eye-watering until that point.
The gate is exactly as it was left.