Let's get one thing straight—the Scout Regiment isn't just essential, it's the entire damned engine of the plot. Without their suicidal, borderline-insistent expeditions beyond the walls, the story would be a repetitive cycle of 'titan attacks, we defend.' They're the only ones actively poking the hornet's nest, forcing the truth to the surface. Every major revelation, from the Female Titan's identity to the basement and the world beyond Paradis, comes because Scouts bled for it on the other side.
Their role creates this brutal tension between humanity's stagnant safety and the terrifying cost of progress. You've got the Military Police cozying up inside, the Garrison just holding the line, and then these lunatics in green cloaks charging out to die for scraps of knowledge. That conflict drives Erwin's gambles, Levi's pragmatism, and ultimately Eren's radicalization. They're the vector for the plot's escalation, plain and simple.
Honestly, the later political arcs just prove how foundational they were. Once the Scouts' purpose gets twisted and co-opted, the whole moral center of the story collapses with it.