One could argue that because of the Job system, "where the hell's my sword" is applicable on the grounds that the question gets answered. In multiple ways, actually, even if the most direct answer is a completely new sword.

The first is out of the question. Who is to decide what makes a true sequel? FFV had Crystals as its sequel. It wasn't a game, but it's not like it lost much by being turned into OVAs, anyways. Furthermore, the swords that Tidus and Auron used are still in the game. You can't make the argument that FFX-2 is a true sequel, and at the same time, ignore FFV's sequel. You can try, but it's hypocritical and smacks of failure and ignorance. Unlike FFV's sequel, FFX-2 provides a raw factory of contradictions, misunderstandings, and a plot that is neither unique nor remotely interesting. I could go on and on about how much FFX-2 makes FFX worse for existing, but we already know my stance on that.

Two only borderline works. The object of Sin is not to destroy the world (though, it is of Seymour's), but to prolong the Dream. If it happens to kill people, it is both not the prime reason, and that it would be feasible due to a revenge factor. If Sin killed everyone, it would be a self-defeating situation.

The third will simply cause QQ. It is not the opinion of the whole FF populace that FFX-2 sucks (even though it does). Furthermore, the Camel that designed the game was also involved in most of the other games, as well; in essence, you'd be indirectly dissing everything else. Not can do. QQ. lol rhyming.

Four and five are like running gags that need to stop. Clever, but the effect would be lost on too many people. FFX-2 isn't even like Marmite, either. This could work, but only if you change FFX's description, too. And that would break the continuity of the descriptions of the Sony-dominated-era FFs.

Six is the only one that makes sense, I reiterate. FFVIII had a gunblade, but it didn't fire. So why call it a gun at all? It isn't! It's a Gunblade. The main focus of the game doesn't revolve around the main warrior, Paine, either. It's Yuna's continuation; example, Steiner had a sword in FFIX, but because he's not the main character, so FFIX's description is as such, "was it a SWORD tail?"