The one and only reason why metformin is getting into trials is as a political lever to get the FDA to acknowledge aging as a medical condition with defined causes for which treatments will be approved. Metformin couldn't really be rejected in the way the FDA likes to reject a lot of things because it is so widely used and has been so widely used for a long time.
The future of human longevity is not tinkering with metabolism to try to push it to be a little more like calorie restricted metabolism. That is a waste of time and effort. It is enormously expensive to carry out the work to try to understand metabolism well enough to adjust it safely. Hell, look at the past 20 years of work on calorie restriction: how much has been spent, how many man-years focused on it, and the research community is still only scratching the surface on how to induce similar effects via therapies.
Yet there is another approach, one that is far more cost-effective in time and money. Repair the causes of aging. Look at removal of senescent cells: in a bare few years, and at far less cost than has been spent on calorie restriction mimetic development, researchers have demonstrated robust effects and reversal of numerous age-related conditions in mice. Unlike metformin and similar drugs, the results are consistent, sizable, and produce rejuvenation in old individuals, not just a slight slowing of aging.
There is an enormous difference between the bad way forward (typified by metformin) and the good way forward (typified by senolytics). In the former we all die on the same schedule as our parents and grandparents. In the latter we can gain decades or more of additional healthy life.