Firstly if you accept we are as human life forms, complex real-time chemical reactions, then in the simplest terms this should show you that extending life should be possible.
https://en.wikipedia.org/wiki/Darvaza_gas_crater
https://en.wikipedia.org/wiki/Centralia_mine_fire
These fires keep burning because there is enough of the right fuel (diet) to keep them burning, their environment doesnt put them out either, even when pathogens (eg us humans) have tried to put them out.

What science is trying to do, is identify what our diet today has or doesnt have in sufficient quantity which can extend life.
Take Rampamycin, aka mTor signalling and the role a keto branch chained amino acid can play in extending life.
There are so many studies on Rampamycin, mTor, Ketotosis, Leucine, and water only fasting and this is just one component of our diet.
https://academic.oup.com/jn/article/131/3/861S/4687154
https://pubmed.ncbi.nlm.nih.gov/22029833/

As we are now at a stage where we can alter what food would normally contain eg supermarkets want marbleised meat, so livestock get fed a Pantothenic Acid (Vit B5) deficient diet as it reduces subcutaneous fat levels and shift the fat into the muscle and around the organs, as well as affecting the nervous system, not a concern for farmers considering the livestocks short life span, but another simple example being popping a multi vitamin pill, it is now possible to alter our diets in drastic ways. The hard part is knowing when something is giving a herxheimer reaction or is genuinely bad for us.

There are a lot of factors to take into account which we just cant measure, the sodium potassium ATPase pump, potassium causes the cell to take up a variety of other chemicals where as sodium makes the cell take up water. Some cells cant handle folded proteins getting inside them for example, yet cooked egg white is a good example of eating folded proteins. We cant measure easily or accurately in-vivo (in the body) what a cell has taken up from interstitial fluid. Thats why radioactive isotopes are sometimes used, or chemicals with an alternated magnetic resonance which can be detected using fMRI scanners. Yet radioisotopes can affect the body, the higher the amount the more obvious the effect as Chernobyl and Fukashima, Radon all demonstrate.
Sometimes, studies are portrayed as being negative to health when the lack of understanding means it should have been portrayed as a positive to health in some circumstances and negative to health in the circumstance they carried out the experiment in, but even the year the study was carried out can sometimes account for that lack of knowledge, other times it just wasnt taught or common knowledge in the scientific community.
Having done a few of my own experiments, I've already discovered some studys portraying negative health effects, but my circumstances have been different. And as much as we may lambaste the food industry, at least we still have reasonably consistent standardised food to eat which will also help the scientific community. In the past like you see in China, a myriad of things would have been consumed as food with sometimes unknown consequences.

Other inhibitors, include what chemical combinations can be made, you'll never see a zinc-sodium supplement because its either too expensive to make, knowledge to make not obtained yet or its just physically impossible to combine the two. So some supplements focus on one element of the chemical combination, and forgets or simply doesnt mention what else its combined with. When you take all the different types of zinc supplements, zinc ions are often a minority to whatever else its combined with, which can also affect the body.

As for it being just for rich people, its also for clever people who demonstrate innovation. Very few things will break the bank, if you have a bank account.