I also think "masterpiece" is a heavy term to throw around but the emotional impact of the this series and the complexity of its narrative as it catalogs a hero's journey from reluctant participant to true believer with an epic story arc can be held up next to most film and historical epics like Laurence of Arabia, Ben Hur, The Matrix, the Original Star Wars trilogy, Dune, Kingdom of Heaven, Gladiator, The Handmaid's Tale (series), The Odyssey (epic poem). His personal journey which leads to his persecution and enslavement, his role in leading a slave uprising, rescuing his friends from the aftermath of a rebel uprising, building the foundations of a rebel army, risking his life countless times and ultimately sacrificing himself to prevent his enemy from having an insurmountable edge.
The series makes the very popular "Avengers" film series look like trite dogshit and does the same for most of the "Star Wars" sequels and prequels so I don't fault people for using the term "masterpiece".