Some people seem to not just survive, but actually thrive on terrible, junk food diets, like the current president, or Warren Buffett (who eats ice cream and mcdonald's for breakfast and 2 liters of cherry coke every day, and is still alive at 95) https://www.businessinsider.com/warren-buffett-diet-2017-10
Diet really does seem to be one of those highly individualistic things, and I'm guessing humanity is in an evolutionary transition from paleolithic type diets to mcdonalds 3 times day, with different people having the genes to thrive on different things. You just have to see what works for you.
Evolution doesn't work over a span of few generations. If humans are evolving to adapt to the modern weetern diet then we won't see that for a very very very long time.
You're just cherry picking examples while ignoring a mountain of literature that shows exactly the opposite of what you're saying.
Yes it can
> Over the past two decades, it has become clear that evolutionary change can be fast enough to be observed in present-day populations (Hendry and Kinnison 1999; Kinnison and Hendry 2001; Hendry et al. 2008; Gingerich 2009) and that it can directly affect the dynamics of populations and communities (Hairston et al. 2005; Saccheri and Hanski 2006; Kinnison and Hairston 2007; Pelletier et al. 2009). Much recent interest has focused on the possibility that so-called rapid or contemporary evolution leads to ‘evolutionary rescue’, whereby threatened populations avoid extinction by adapting to an altered environment (Barrett and Hendry 2012; Gonzalez et al. 2013).
https://pmc.ncbi.nlm.nih.gov/articles/PMC3894905/
In any case, it's moot as by and large the westeren diet is not good for the population, exceptions are simply that.