I don't think that everything that Byrne did was horrible, which on this board is probably heretical.
I don't like to use terms like "all" or "never." Most of the time because they don't apply. Even writers that I despise have written and done at least one thing I like that was worth reading: Ron Marz wrote a wonderful Superman Elseworlds where Superman was raised by the Guardians and became a Green Lantern; the idea was not the most original in the world but the joyfulness of the execution was. Among other things, it featured Hal Jordan battling Sinestro, with Jordan creating a green gorilla and Sinestro creating a yellow octopus with a brain in a jar to wrestle each other. Mark Waid's JLA: YEAR ONE was a very worthy, detail-rich miniseries. Frank Miller's DAREDEVIL was absolutely wonderful the first few years until the Hand showed up. And best of all, Jim Owlsley, the infamous weasel that wrote the abysmal, destructive EMERALD DAWN, to my great surprise and delight, is one and the same man with "Christopher Priest," who wrote BLACK PANTHER, the greatest comic of the 1990s besides Kurt Busiek's AVENGERS and Gaiman's SANDMAN.
Every terrible writer I slam has done at least one story that was worthwhile.
There are three exceptions to this rule.
Two of them are Chuck Austen and Warren Ellis (and at least Warren can be funny sometimes).
The other is John Byrne.
Hey, that's one I rather liked. Given the wildly inconsistent ways Namor had been depicted, across many magazines, dating back to 1939, it wasn't a bad fix.
It was a terrible fix, for the reason that Namor is made an interesting, complicated villain because he
isn't in the wrong. His motivations are complicated, based on honor and pride, and a legitimate, undismissable complaint with humanity: our exploitation of the seas to dump garbage, pollution, and as a place to hold our wars. Namor was conflicted, he was brash and proud. He is truly one of the five greatest villains in comic book history.
Byrne's retcon that Namor had just been "crazy" all along with a blood disease removes the one thing that makes Namor effective: sympathy. Namor is a villain because of his heroic qualities, ironically. If Namor was not responsible for his actions, the thing that makes him a compelling foe, that his point of view is not an invalid one, is eliminated.
Yes, Namor has been characterized inconstantly by bad writers (though it should be noted that those that wrote him the longest: Stan Lee, Thomas, and Steve Englehart in SUPER-VILLAIN TEAM-UP, gave him a fairly consistent characterization). But this abysmal Byrne concept is destructive to the character because it makes his grudges and pride, the very things that DEFINE Namor - to be something alien to who he is.
There are ways to incorporate bad characterization into a character by looking at all the pieces and seeing what works. Look for instance, at Kurt Busiek's examination of the actions of Wonder Man in his AVENGERS run; he placed Simon's out of character selfishness and ego into a context that made his actions make sense.