I remember reading a few articles and threads about this topic back when Wizards of the Coast began including the Legacy Content disclaimer on some of the old TSR pdfs. I remember thinking "we aren't losing anything. It's the same work, they just rubber-stamped a disclaimer on the first page...and a very milquetoast disclaimer at that. Nothing is going anywhere."
I still feel the same way, more or less. Republishing the Dark Sun campaign setting, for example, isn't going to un-publish the original 1991 books. Some people are going to prefer the original version, for various reasons, and they will still be able to find, purchase, read, and play them. Nothing is "lost."
But there is plenty to gain. Continuing the Dark Sun example: by updating it to the new rules set, updating the artwork, and updating the tone, they can introduce an entire new generation (and a much larger generation) to the setting. Releasing Dark Sun without updating it would make Wizards of the Coast look tone-deaf and out of touch, it would tarnish the product line, and it would alienate an entire generation of potential players. For better or worse, Wizards of the Coast is the TTRPG industry leader and they have to act like it. (And let's be honest: they already don't have a stellar record in this department, and all eyes are on them at the moment.)