Thing is, you can't overthrow HERO without the Level 5 Captains and their squads gone. They're the main force of HERO. They share the same ideology. If Dexter were to die, the captains and Xanthe would take over until Tiffany is old enough. Hence, HERO would still exist. The guy said he wanted HERO gone, so the captains and their squad would have to be killed or sent away in his scenario. If not then they wouldn't bend to some random guy's demand.
As for the last part, that's my point. If HERO can barely stop the monster attack, with all of their power, what makes anyone think they'll do better if HERO disappeared? Especially with the Apostles on their way right now. It's basically saying extinction is fine as long as I destroy HERO and the World Government first. It sounds more like a ego thing because peace isn't going to happen with HERO, World Government and SIN gone. Only thing gone is humanity and the world itself.
Does make me wonder how we are to actually DEAL with the Apostles, let alone the Arbiter's if they "resurrect".
So far they have found 2 of their corpses but I believe (could be wrong) Zack beat ONE at Lvl 4 or with an unperfected Xanthe Serum (which is why it took him so long to recover?)
Regardless, even if they DO beat them those are only their Vessels. They'd still have to ACTUALLY kill their original selves. I have no doubt Bernhardt, Malik, and Deus could destroy them, but the rest? That might be a challenge...even if we assume ALL the Captains could take on ONE that's only 5 Apostles out of 12 they have to beat, and that's disregarding other Uber powerful S Class Monsters like Aglaecwif, Valravn Prime, Rebis, etc.
Some of the Lieutenants could also put up a fight too obviously, like Henri, and maybe Lisa & Kira, possibly Elijah (not enough known feats). But I doubt Alexis, Clark, or Nicolette would be able to KILL the Apostles, they'd have to work on the S Classes and Ancient S Classes.