But that's just it: G'raha's destiny isn't to seal himself inside the tower and bring "the light of hope" to the future. That's just his vague interpretation of Salina's wish, and probably brought on by his feelings of powerlessness after the World of Darkness, prompting him to invoke his own Heroic Sacrifice as the only way he thinks he can even the score.
Unlike Desch's destined sacrifice, the urgency is an entirely artificial construct. The "danger" of the tower is completely undefined, since the Calamity itself was caused by a glitch in the Dalamud uplink, and the value of the "light" it can provide is never weighed against G'raha's own value as a human being to the people of today. Pardon me for thinking that the Encyclopedia Allaganica in his head (not to mention his own skills and knowledge) can be worth far more than cheap, clean solar 1000 years from now. But, again, G'raha simply forced the situation for reasons unknown.
Now let's look at Desch. He thought he was destined to die repairing the Tower of Owen to keep the Floating Continent from crashing, and said his final farewells to the party before throwing himself in the reactor. In reality, not only did it not prove fatal, his willingness to sacrifice himself was proof that he bears a soul of light, and because of that he was still needed as one of the five who could save the Onion Kids from Xande's trap in the Crystal Tower, one which was "destined" to succeed. Sounds like a pretty big difference between his perceived destiny and his true destiny.
For the FFXIV edition: think Archons. Aside from the keyest-of-key MS NPCs, G'raha is the only character to bear the marks of the Scions; a fact that has yet to be touched upon in any NPC dialogue, but with what we know shouldn't be a complete accident. Shouldn't. We still have no idea what those marks really mean either way, so while G'raha seems like a simple red herring for now, the coin is still in the air.
But "destiny" is merely the name characters in a story give to the almighty will of the authors who shape their world. There's a certain amount of hubris to playing along with what you think your destiny is, versus simply following your own chosen path and letting the world sort itself out. Rebelling against destiny is also a way of following along with it, and that G'raha at present is a fairly apt double for Serah Farron is perhaps a sign that we're going to end up with more of the latter.
(Also, while I don't like bringing it up, the devs can, at any time, make CT into a requirement for future quests, not unlike what happened with the Sunken Temple of Qarn in 2.1. It would be a long content gate, but up until now they've shown a commitment to keeping "old" casual content from becoming "obsolete" content. It's possible for it to become a gate, and also just as possible, with timeline conceits in place, that we see a continuation in the Main Scenario that simply assumes you've done it before then (looking at you, Estinien). We have no way of knowing either way until it happens, so there's no point in using "optional" as the sole crutch of an argument.)