I agree that diet and lifestyle interventions are limited by patient compliance, cost, access, etc. I also agree that in many cases those are cop-outs. You don't have to buy local organic veggies from the farmer's market to eat well. You can buy frozen veggies, which are cheap and probably about as nutritious since being shielded from heat and light slows micronutrient break down (e.g. folate).
Meat is generally more expensive than fruits and veggies, and eating too much protein may increase the risk of some diseases. So, you could also save money by eating more plants and less meat.
Personally, I like to focus on diet and lifestyle things that are basically free. For instance, just not eating late at night can have positive effects on some people.
Some (but not all) RCTs show only eating during a limited window of time during the day (e.g. 4-12 hours) led to significantly greater weight loss, for instance. (However, only one of them showed 5%+ weight loss, which I believe is part of the FDA's approval criteria for weight loss interventions. That one involved eating only during an 8 hour period per day.)
In practical terms, these time-restricted eating interventions basically come down to eating an early dinner and not eating or drinking anything but water and prescribed medications until breakfast the next day. Anecdotally, this change seems to have decreased a relative's BP and fasting glucose so much that the dosages of their medications have had to be lowered several times. They've also visibly lost weight.
Also, I believe
LizzyM mentioned not drinking sugary sodas. That's a great one that actually saves a little money. I'd add to that not drinking sugary fruit juices every day. In the Framingham Heart Study cohort,
people who drank one or more fruit juice servings a day tended to have significantly smaller brains that weren't as good at burning sugar for energy. Just a correlational study, but still. Daily fruit juice drinkers also tended to have significantly worse memory of events.
You can also turn off lights at night, which saves money and can
increase your body's production of melatonin.
Melatonin is one of the only antioxidants that can detoxify hydroxyl radicals, which are one of the most damaging kinds of reactive oxygen species.
Bad diet and lifestyle choices can dramatically increase people's risk for specific chronic diseases. Good diet and lifestyle choices can help prevent chronic diseases or maybe even slow their progression in some cases. However, they are not first line treatment for anything (with a few exceptions involving micronutrient deficiencies). Medicine is.
Sorry for any errors in this message, and for the length. And of course, none of this is advice of any kind. Just trying to show that some of this stuff is accessible to most people.