פירוש על עבודה זרה 9:11

The Talmud now emends the statement with which this section began. Had they not sinned, Jews would have lived forever. While they would have procreated, those Jews who were at Sinai would eternally overshadow those Jews born later.

The idea that had they not sinned Jews would have lived forever is problematic—the Torah contains laws which assume that people would die. The laws of levirate marriage assume that married men die and the laws of inheritance clearly assume that people die.
The answer is that these laws were written conditionally. They were given under the foreknowledge that they would be relevant only if Israel did not sin.