> If you are relying on requirements.txt merely pinning versions and not transitive dependencies, then yes, you are holding it wrong.

Even then, if you have the original environment, you may very well not need to re-create it. If you do need to re-create an equivalent for a newer Python version (EOL versions of Python don't just spontaneously vanish, but maybe you upgraded your system or something, or better yet maybe you're planning in advance for that) then you generally can easily create the needed requirements.txt (or, for that matter, a PEP 751 lockfile) from the old environment.