I upload my pictures with quality q=X. The next day I download a picture. Quality is q=Y, where Y < X. One year after, I download the same picture. Quality is

I upload my pictures with quality q=X. The next day I download a picture. Quality is q=Y, where Y < X. One year after, I download the same picture. Quality is q=Z, where Z < Y < X. So my pictures uploaded to Google Photos suffer from decay, whenever Google chooses to degrade quality. None of your "not unreasonable" scenarios are involved.