Non-existence could be, if you are in another namespace, or when you are using multiple sessions via a shell-script for example
I dont see that this has any relevance to setting the random seed of the active R session. If you pull up the same script in another session, for that script to be fully reproducible (same outputs if processing same inputs), it will be possible to set the seed, and indeed necessary. Again, if your seed to set, is an R integer, there is no failure case.
So what is your case for reproducibility?
The ability to stand behind my work and justify it, on a different day than the day I originally created it.
if thats not a requirement for you, forget seeds altogether, but
a) I would find it maddening to rerun a script and find the numbers jumping all about from one run to the next, it would make debugging more difficult
b) my work is generally not a one off that no-one would challenge, so reproducibility is a must.
R is not a religion, indeed, and you dont have to use set.seed. and you dont have to make your work reproducible, but we can infer that this would likely not be a) science, b) commercial work. If its a pure hobbyist thing, fine, have my blessing to throw reproducibility out of the window. on the other hand, its best to have good habits....