How long do Breast Implants last?

Hi, I got breast implants over 15 years ago and to be perfectly honest, my boobies still look and feel as good as new. Except I know implants aren't supposed to live forever. My question is how long do these implants last and would it be dangerous if I never replaced them. I don't see why I would want to change them so long as they still look good. Is this just some scam to better line the pockets of plastic surgeons or a serious health concern?