The common defense I hear from those who do like the ending is that it teaches a "realistic" message about the unfairness of life, and how even if you try your very best, you won't always succeed. To be fair, this is, unfortunately, true, and it's an issue that has been tackled in other media too, probably most notably Star Trek TNG with Picard's well-known quote "It is possible to commit no mistakes and still lose. That is not a weakness. That is life."
So if TNG can do it well, why can't Dragon Ball? Simply put, because the tone of those two shows is obviously very different. Actually, it'd be more precise to say that TNG is different tonally than most other shows in its genre that it gets compared to. TNG handles its topics pretty delicately, with a balance of emotional appeal and logical reasoning. It is hopeful and forward-thinking without denying the shortcomings of humanity, and addresses those issues in a way that people can accept while not being all doom-and-gloom either. That's perfectly fine for TNG and it's what makes the show so memorable and why it resonates with so many people.
However, Dragon Ball, and for that matter, many other Sci-Fi/Action shows, would not be able to handle this topic the same way, and it's largely because, well, if they did, the story would absolutely suffer for it, like I believe it does in Super with the Zamasu arc's ending. If you extend the logic of the ending of the Zamasu arc is good because it's "realistic", then by extending it to its logical conclusion, you could say this about pretty much any good/hopeful ending for any well-known media franchise. I guess in Star Wars the heroes should all die off horribly despite their best efforts, because their chances of success were very small anyway so it's more "realistic" to show it that way and it teaches the audience a "good lesson about life".
This isn't a post-apocalyptic drama, a grimdark horror, or some other genre that is more well equipped to deal with more "grounded" takes on hope. People watch shows like Dragon Ball because the characters, despite all of the odds being against them, end up succeeding in their goal in the end. Sure, obviously they may suffer major losses on their way to achieving said goal, and if you went the other extreme and got rid of these then that is also of course, bad. That is because those losses on the way make the eventual triumph all the more satisfying, and that is what people want here: satisfaction. An ending where all of the hard work, blood, tears, and suffering ends up not amounting to anything does not make for a satisfying conclusion to a story with the tone that Dragon Ball is known for.