DC characters were mostly created at a time when their costumes, powers and gimmicks were more important than the personalities and lives of the actual characters. Marvel characters were created at a time when that was reversed. The original point of Flash and Green Lantern was nothing more than a guy who could run fast and another guy who could shoot light from his ring. But the original point of The Thing and Spider-Man was that one was a relatable everyman and the other was sad and angry that he looked like a monster although deep down he was a teddy bear.
That’s the biggest reason DC characters work so much better whenever they are given the legacy treatment or riffs on their characters (like Supergirl or Batman Beyond) are created. It didn’t matter who was Flash or Green Lantern because the characters were built around a power not a specific person. The biggest backlash against changing a Flash was when Wally stopped being the Flash because he was the first one who’s personal life was made as if not more important than all the superhero stuff. Even the backlash when Kyle Rayner debuted was more about HOW Hal was written out than people being angry that Kyle was replacing him.
That isn’t the case with MOST Marvel characters. Legacies do not work as effectively for them as they do for DC characters. Captain America is one of the few exceptions because he was created in the same time period as most of the big name DC characters, but even then he’s only ever been temporarily replaced and each time was met with initial backlash.
Marvel characters like the DC ones can have great riffs too, like Miles Morales or Spider-Man 2099. But they are not actually REPLACING the mainstream versions of the character. They’re seen as more separate and their own thing, even Marvel acknowledged that since Miles Morales only ever replaced the Ultimate version of Spider-Man who Marvel and most fans just saw as a separate version who wasn’t the main or ‘real’ one. It’s also why they were chill with killing off Ultimate Wolverine or making Ultimate Captain America president. They could experiment and go in different directions because it didn’t matter that much because the Ultimate universe was never the actual Marvel universe people cared about as much.
However few Marvel characters work in the long term as legacy characters. At best you can only do temporary replacement stories for the characters where the original was always going to come back anyway. Even then those stories WORKING is hit or miss and Marvel fans tend to react negatively against them. It happened in the 80s, the 90s and the 2010s, it’s just how it goes.
Because the Marvel characters were first and foremost built as people first and powers second.
Yeah there have BEEN lots and lots of attempts at legacies and riffs on character (e.g. all the Spider People from Spider-Verse) but few of them succeed financially in the long term or are creatively very good in contrast to DC who can make more legacies, alternate versions and riff characters work despite not necessarily having AS many as Marvel have tried.
Ultimately for the majority of Marvel characters, the original characters ARE that identity. Rhodey was an awesome temporary Iron Man and an even better War Machine. But Tony Stark IS Iron Man. Jane Foster and Eric Masterson were decent temporary Thors and Eric was an amazing Thunderstrike. But Odinson IS Thor. Miles Morales is a decent alternate riff on Spider-Man and a great Spider-Man for his own universe. But Peter Parker is THE Spider-Man. In all cases every successive version is premeditated upon being a response and contrasting to the original as opposed to being their own thing.
It’s why people barely talk about Johnny Storm being the second Human Torch. The first one was an android with flame powers. Johnny though was a personality who happened to have flame powers.