It's a pretty big disaster that wealthy nations have hungry, homeless, sick people that are considered acceptable for the sake of profit and they make benches like this so people can't even have a nap.
Some of it would remain and protrude and have to be sanded down to avoid injuries, then possibly painted or otherwise treated to prevent rust. The only viable solution to this problem is ensuring that the right kind of benches are installed in the first place.
Well, I mean, you could grind it down smooth with said angle grinder + bring a rattlecan of rustoleum for after. Though I suppose that does increase your chances of getting caught in the act, I doubt it'd add that much time.