Don't think that dentures are suppose to last forever because they don't. If you do or don't have any teeth your mouth does still changes slowly as you get older. The average jawbone, if you don't have any teeth is about 1/3 mm per year.
If you have osteoporosis, you have a greater chance of having jawbone loss. When the jawbone shrinks, so does the ridges or the gum tissue. Gum tissues or the ridges can shrink about a quarter (14) of inch in ten (10) years.