I don’t think anybody actually believes living longer than 120 is impossible. Clearl;y there are a few people in modern history that lived longer than 120. As well as people in Tanach. Yishai was after Moshe Rabbeinu and the Medrash says lived 400 years (though this may very well be a typo/misunderstood abbreviation as some have pointed out).
Interestingly many scientists believe we are not designed to live longer than 120 (generally speaking) Aging is a real phenomena and the body simply breaks down organs stop functioning properly etc etc. Also worth noting is Longevity hasn’t increased much in the past century. Life expectancy has increased, but this is primarily due to increased survival in infancy which has a large impact on average life expectancy
(Eg if you have a society with 10 people 5 live to 90 and 5 die at age 2 R”L The average life expectancy for that group is 46. IF the 5 survive to say 50. The average life expectancy of the group is 70. But in both cases the longevity of hasn’t changed much)
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