A large system update does more than copy files. It downloads, verifies signatures, stages replacements, and then swaps thousands of components during restart.
Progress percentages are estimates, not measurements. A figure can sit at the same number for a long time while a slow verification step runs with no visible activity.
Genuine failure looks different: an error code, a rollback message, or a machine that restarts repeatedly without progressing. Those are worth acting on.
The dangerous move is cutting power during the swap phase. That leaves a half-replaced system, which is far harder to repair than a slow update.