And this is an example of "where is the evidence" logical fallacy. In complex systems, such as the human body, the only evidence of safety is time -- a very long time. The Stevia plant (though not the same as extract) has been used for centuries. Aspartame was (accidentally) created in the lab in 1965.

If you are going to mess with complex systems, such as feed yourself aspartame, vape, give pregnant women a synthetic sedative and medication for morning sickness etc., you do it for a high potential gain that offsets potentially huge downsides. Benefits of aspartame are close to nil; if anything it probably makes things worse because people think they can consume more ultra processed foods sweetened with it because such foods have "fewer calories".