Upon a close listen, you’re right! Tenma does say さらば rather than さよなら. Some quick research indicates that さらば is archaic/poetic, and is often used in anime and manga as a result but very rarely in day-to-day speech. (Think Shakespeare — it’s beautiful in plays and movies, but if someone started talking to you on the street like, “Foresooth!” you’d find them ridiculous.) The meaning is the same as さよなら, though — that is, a final farewell (emphasis on final) — but the use of さらば instead in this scene adds a little bit more of a cultural “omph.”
I also did some research into the implications of Tenma ending his sentence with だ, and it’s worth noting that だ is typically considered more casual than です, though both are used at the ends of sentences. So Tenma is speaking casually to Atom — no surprise, since this is his son. But だ is also used for statements directed at yourself/your feelings, so it could be Tenma’s way of dismissing Atom’s presence all together, or saying that he doesn’t care anymore. (This is a bit of a reach, since I’m fairly certain he’s always addressed Atom casually, but this is an intense scene and I am fascinated by the nuance of だ vs です, so I can stretch a little if I want to.)
Either way, the takeaway is the same — Tenma’s final goodbye hits much harder in Japanese than in English, even with a single word.
Finally!! Finally an answer for this!! I wasn’t miss-hearing! This is what Tuxedo Mask says when he leaves the scene in Sailor Moon!!
when your mutual reblogs something with a full page of tags its like. girl (gender neutral) i am filling my mug with coffee and reading this like the morning paper. i am so interested in your thoughts on this post. i love you.
