It mostly comes up when the character is a POC or female or both. Even when it’s clearly temporary (like Sam Wilson as Captain America or Jane Foster as Thor).
Bucky was Captain America and was largely accepted because he was white…never mind that was previously a brain washed assassin that had murdered a lot of people. Falcon became Captain America and the usual crowd totally railed against it. Or worse still look at Riri Williams who was Iron Heart in the books and not Iron Man, folks are still making videos about Riri to this day, conveniently ignoring the fact that Victor Von Doom was parading himself as Iron Man. It was so bad with Riri that there was even a petition against her D+ series a while back.
Parts of the comic book audience only accept replacements when they’re white, straight and male. It’s pretty awful but it is what it is.