Firstly, you got into an argument with someone on YouTube. You may as well have dressed up as a knight and marched into 4chan, and demanded they end their wicked ways. Someone is going to get butthurt. Trolls of ultimate, that is internet.

And Final Fantasy isn't a dying series at all. He was wrong on that completely. The last game wasn't what most people expected and they didn't really like it, sure. Some people may or may not drift away from liking the series, but the series will always find new fans. Who the old school fans will dislike because they won't shut up about how hawt Tidus is, and how they want to cosplay as Lightening and Serah but can't because they're too fat. But they do it anyway and look ridiculous.

But yeah. Final Fantasy is not dead. You don't need to waste time on YouTube arguing about that.