"This isn't important", "this doesn't have an impact" and "I won't change it" without giving a reason are always wrong.
But the code review policy in OP's company looks weak: It's the onus of the author to convince the reviewer, not the other way around.
A good version of the first two responses could look like: "That would have a small impact because of X and Y, and it doesn't seem to me that the extra complexity in the code would be worth it". If the reviewer isn't convinced and you still don't agree, propose to open an issue to measure the actual impact, so the current change isn't blocked.
But the code review policy in OP's company looks weak: It's the onus of the author to convince the reviewer, not the other way around.
A good version of the first two responses could look like: "That would have a small impact because of X and Y, and it doesn't seem to me that the extra complexity in the code would be worth it". If the reviewer isn't convinced and you still don't agree, propose to open an issue to measure the actual impact, so the current change isn't blocked.