First let me say that, at the moment, I tend towards the theory that travelling back through the dimension of time would actually bring you out in one of an infinite number of universes. I also consider multiverses unlikely, and I simply cannot accept time travel on a human scale, although on a subatomic level, it might be possible.
On with the show:
Perhaps it's me, but I don't see why "the means by which that alteration can be carried out will never be invented."
Niven's proposition seem to indicate that if the past were alterable, then a time machine could not be invented. This seems like a non-sequitur - there is nothing to say that the invention did not take place at sometime in the future, (what a strange use of tenses) and the inventor carried all details with him and returned with them.