20% increased risk of death or disease over the span of a generation is not worth getting excited about? For someone who's healthy, it may seem insignificant when the baseline risk is low, but for someone who develops a chronic disease, in hindsight a 20% reduction may be very appealing. Minimizing regret is important.

As for scientific validity of those studies, doing proper scientific studies of diet is so difficult it might as well be impossible. What kind of person is going to volunteer to have someone else dictate their diet? What kind of person is going to comply with that diet long-term? If you want to do a blind study, you have to turn all food into unidentifiable mush. You can't have people seeing broccoli florets or pork chops.

There is a proposed theory for why red meat has those effects: Neu5gc consumption causing persistent low-level immune response. Less conclusive dietary studies have noted a difference, but without doing a scientific study giving one group Neu5gc supplements and another placebo, how can we know? Ethics boards might have a problem with that, since Neu5gc can be demonstrated in vitro to trigger an immune response.

Perhaps the most general and plausible theory is that it has nothing to do with inherent unhealthiness of red meat or meat in general. Rather, more meat consumption means less dietary fiber, fewer fruits and vegetables, assuming the same total caloric consumption. Meat consumption probably also correlates to excess calorie intake, which is a risk factor for a variety of chronic diseases.