Also how do people know it's approaching it's final arc??
I think some people over-reacted (not so much here, but other places) to the news tbh, it was pretty obvious we were coming toward the climax. Technically, their 'final mission plan' to overthrow the capital began way back when they discussed taking out Borick in chapter 28, even before the battle with Bols, Kurome and her puppets. The plan was to take out the Jaegers, take out Borick, then the Revolutionary Army would incite multiple uprisings and attack the capital, using the Path of Peace religious group that Borick had infiltrated and defectors within the Empire's army to aid their effort in making ground as fast as possible. Then Night Raid attacks the Capital while the military is stretched thin and takes out the PM. Well, they've weakened the Jaegers, taken out Borick. All that's left after this mission to save Tatsumi is begin the final uprising. Who knows how long the final arc will last though, there's still a lot of stuff to cover/deaths to happen.