I didn't even need to stand my ground at all. He just said it wasn't allowed but he'd let it through this time, and asked me to check it in next time. The whole conversation took about 5 seconds I think.

I'm quite surprised that you (wrongly) deduced from my post that I argued with him. I wouldn't have bothered arguing anyway, because frankly, I can't be bothered.

But if I had argued and if I had had a good reason to believe that I was right and he was wrong (in this case I have no idea but assuming this is the case), it's not for you to expect that I do not argue my case so that you can be in the lounge/gate 2 min faster, is it, provided that I do this in a reasonable manner?

Edited to add: in fact, I have just found an official document from the French government, and the rules pertaining to scissors are the same - ones with blades max 6 cm is permitted.
So are you saying that you would prefer to deprive other passengers of their right to argue their case when they are in the right just so that you can get through the security faster?