I think it's important to distinguish between "life expectancy gains have slowed substantially" vs "meaningful longevity increases are not reachable".
A huge fraction of deaths in the developed world are from "lifestyle diseases" from obesity, poor food choices, sedentary behaviors, alcohol, tobacco, etc, all of which we could improve. We eat too many highly processed foods, added sugars, etc. We have places without infrastructure for clean water. We have gun deaths and traffic deaths, and we have bad gun laws and car-centric communities. We have flooding/hurricane/heatwave deaths and we have a climate-denial public policy. There are _so many fixable things_ that shorten people's lives, and we'd all probably also live happier lives if we fixed them.