In the late 90’s I assembled my 15th set almost entirely from packs, so that gave me a lot of experience with the “aged gum” issue you’re asking about. The gum was originally coated with powdered sugar which made it less sticky, which may or may not hold up over time, depending on how the packs are stored. I got lucky - in nearly every pack, the sticker facing the gum was unaffected. In some cases the gum faced the back of a sticker and that was more likely to leave a slight imprint. The glossiness of the sticker fronts seemed to make the gum less likely to stick.
Conversely, in a later experience with a full GPK box, the results were not as good. But in that case it’s 5 stickers to a pack I think, so even under the worst circumstances it only affects 20% of the stickers, as opposed to potentially 50% of Wackys.