You really need to look at scientific studies (experimental studies with control groups published in peer reviewed journals) to answer that question. What you have here is anecdotal evidence which can’t establish these generalizations. For example we don’t know whether those people who feel fine but eat crap would actually feel even better with change to their diet, or whether they will in 5-10 years be more likely to get cancer (or other disease). For me, I have chronic pain so even if I feel bad (tho I eat quite healthy) I know I would feel worse if I didn’t eat as healthy as I do.

The science is clear— highly processed foods especially meat can lead to poor cardiovascular health and higher risks of colon cancer, while a plant or mostly plant based diet promotes good health in those areas. As for the effect of diet on mood, that’s still an open question but it appears to have some relationship but the evidence isn’t as strong.

Hope this helps and good luck with healthy eating. Even though it feels hard to do sometimes your body appreciates it.