I just have to say, the speculation into the potential global blackmail by the Princesses makes me chuckle and scratch my head.
Beyond the fact that everything we've been presented with suggests that the Equestrians are a peaceful people who will likely jump at the chance of joining our trade based political exchange (once we get around the whole "monster infested barriers" thing), I don't actually recall any in text commentary suggesting that they actually control the Sun and Moon.
They're almost certainly immensely powerful, and likely have a life span measured in centuries if not millennia if not infinitely, but present evidence suggests they're either going to be excellent trade partners or
very slowly open their nation up due to the complicated political and militarized world they're stepping into.
Not, ya know, become supervillians threatening the entire world with Eternal Night if their demands aren't met