Sword Art Online the Movie: Ordinal Scale — Ending Explained

The Twist: The 'game' of Ordinal Scale is a facade. The boss battles are not for entertainment but are set up to trigger fear in SAO survivors, activating specific brain patterns that allow the Augma to scan and extract their memories of SAO. This process can kill them.
The Ending: Kirito and friends dive into the Ruby Palace (Floor 100 of the original Aincrad) to defeat the true final boss. By doing so, they give the AI Yuna the power to defy her programming. She returns the stolen memories to the survivors but fades away as a result.
Post-Credits: The technology and data from the incident are not destroyed but seized by Kikuoka for 'Project Alicization,' directly leading into the next season of the anime.

Some critics view the film as a critique of nostalgia culture. The villain literally mines the past (memories) to reconstruct a dead ideal (Yuna), which destroys the present well-being of the survivors. This can be read as a warning against being unable to move on from the 'glory days' (or in this case, the trauma that defined them). Additionally, the film can be interpreted as a deconstruction of the 'gamer' identity; Eiji represents the toxic side of gaming—using cheats and gear to mask insecurity—while Kirito represents the 'pure' gamer who eventually learns that physical reality is just as important as digital skill.