I finished watching Zambot 3 this year. Zambot 3 is a short 1977-8 super robot anime, directed by Tomino (for the show’s salient features, see here and here). It doesn’t pull its punches, and I liked it, even though it wasn’t very animated at times. Before you ask, no, there isn’t a ‘well-‘ missing from that last sentence.
Zambot 3 ends well. I sat where I was and didn’t want to do anything for a bit. The rest is spoilers.
Animanachronism 
Like you, the ending dumbfounded me. I daresay there’s no other mecha show that did this to me.