Hi jege41,
Sometimes it can seem easy to see “patterns” on the internet that may not indicate patterns at all.
I have talked with the supplier of these toppers in person (Literie Laurier in Quebec) and they sell hundreds of these toppers themselves and through Costco and they don’t have any unusual softening issues.
In post #1 here it is clearly an issue of the topper following pre-existing dips or soft spots in the mattress.
The pictures in post #17 here as well (which were sent to me and I just added) seem to clearly indicate that the topper issues are related to the mattress underneath it and the bigger concern was that the topper wasn’t listed correctly on the Costco site (which has since been corrected).
None of these are to minimize your own softening issues but the two Costco topper examples you are mentioning are not about the topper softening IMO.
Phoenix