In other words, nuclear war is the only thing that would stop human progression at this point.
"And if not contained, It can destroy everything in Its way."
That proves one thing: we caused our own demise.
Can not someone do comics where mane 6 and princess actually cooperate ?
In one of your chapters, the villain say that power is like vine, the more time passed, the better it is, he also mentioned that elements are like hundreds years old, but Nightmare Moon was defeated 1000 years ago, and we do not know how many hundreds of years passed before they were used again Discord, and also no one know how long they existed before Celestia and Luna found them, so yea, I would give elements 1500- 2000 years at least.
Also, by that logic, would not it make Celesita and Luna who live so long far more effective to deal with main villain then mane 6 who got elements 2-3 years ago, I know that they are not element bearers any longer, but if elements were reason behind their ascension ( like they played major role in Twilight ascension in season 3 finale ), it mean that Celestia and Luna have are still major players.
Since everyone expect mane 6 to defeat villain, having Celestia and Luna assist in defeating it would be nice unexpected surprise, after all the best ending is unpredictable ending, so feel free to give Cadance and Shining Armor a part in defeating the villain, or even make Twilight ascent thanks to elements as a power buff again final boss, and you can even thrown that not just mane 6 but also many more ponies in Equestria will have to contribute in defeating him ( after all better than 6 friends working together is a kingdom working together ), I know that I am just giving weird ideas out of blue, but feel free to surprise us.
I am counting on you.
PS: From my perspective mane 6 are like glass canon, they can KO villains, but without elements they are still normal ponies who can be defeated, and it was proved where mane 6 beat 100 changelings but were overun in the end, proving that mane 6 are not one pony army. Alicorns seems more of like a shield and sword, they are strong and hard and can take a punch. So the reason why I want to see mane 6 cooperate with princesses is because 6 ponies can not keep burden of entire Kingdom alone.
So yea, from my point of view moving sun and moon seems to use only fraction of their power ( and Discord could use it with even less efforts ), so I am sure that they have far more power to spare.
Also, considering that Twilight become alicorn, do you plan to use this in any way or is it to late for that ?