Exactly. They haven't had to actually destroy any realm yet, because they don't need to, they threat of doing it is all they need.
That is incorrect. There are realms around that have not given in to the Zuma's threats. There are realms that would be perfectly willing to go toe-to-toe with the Zuma. We might win, or we might lose. But we'd still do it. We just haven't had to do it yet.
And their physical characteristics, according to the mechanics of the game (the only thing players have to go off of), says they would have no problem doing so if they needed/wanted.
How do you know that? Have you ever faced them in battle? I.e. have you ever actually assembled an army with the express purpose of attacking the Zuma, and then gone ahead and done so? Have you ever assembled a coalition of four or five realms and tried to stand up to the Zuma? Or have you always rolled over and done whatever they wanted?
Now, if you stand up to the Zuma, you just might find yourself wiped out. We don't know what will happen, because no one has ever tried to do it before. Maybe the Zuma will back down. Maybe you'll fight and win. Or maybe you'll die in painful agony as the Zuma burn your realm to cinders. But until you actually try it, you can't say "We
know this will happen".
Oh, you mean the same CE that is quite widely regarded as being the cause of deadlock and anemia on Atamara?
Sure. And if you don't like the CE example, then substitute Perdan. Or Sirion. Or Arcaea. Or, at least until the last invasion, Enweil.
Also, the idea of "Whoever has the Zuma on their side wins" is factually incorrect. Kabrinskia had the Zuma on their side. And the Zuma wiped out Kabrinskia's army. Kabrinskia didn't win that war, they ended the war in a stalemate. And perhaps I'm not remembering correctly, but as far as I can recall the Zuma have never stepped into a war and given the victory to one side or the other.