I mean, yes, marketability is a big part of the reason that death is such a revolving door in comics, but another big part of it is that most comic writers are actual children. They’re all:
“I’m not interested in using these particular characters at this particular moment, so it’s inconceivable to me that anyone would want to use them ever. Let’s kill them all to prove that my pet villain means business!”
…or:
“I have a weird grudge against a fictional character, so I’m going to include them in this story even though their presence contributes nothing to the plot specifically so that I can shit all over their characterisation and then give them a pointless, humiliating death!“
… or even:
“I’m so egotistical that I think my interpretation of a character that’s been around for the better part of a century and has passed through dozens of writers’ hands should be the absolute last word, so when I’m finished with them I’m going to deliberately break my toys by killing them off and fucking up their supporting cast so badly that it will be completely impossible to use them in the future without totally rebooting the franchise!“
… and then the next writer is like “actually, that never happened”, and we start the whole business over again.