I just finished the game and it just booted me back to the main menu after I finished the last boss, with no cinematic in sight. I'm playing a vanilla gog version. Any thoughts on how to fix this? Ended up youtubing it instead, but yeah.

There's a cutscenes directory, cs3.avi being the ending video. Did all the other videos play for you? You can consult the FAQ, there's a last resort fix for this.

Yeah, the first two intro cinematics worked for sure, as well as the shuttle videos, including the one later in the game (if I recall correctly). That's why I was so confused as to why the outro cutscene didn't trigger.