You're looking at this from the wrong angle. The dementia risk and longevity have overlapping factors. Lifestyle choices correlated with shorter lifespan are also correlated with earlier dementia onset. They're not perfectly proportional, but getting earlier dementia and also living shorter is lose-lose.

Technically there's an extreme version of this where someone's lifestyle is so bad that they're unlikely to live past 60, but the last 10-20 years of that life would not be fun.