I would actually agree with that, except that people do play more then one class in this game. Thing is though, there aren't looting restrictions in Diadem, so you can conceivably free farm up gear for alts faster there then in Void Ark.
200 gear just doesn't really work as a reward, when 210 gear can be free-farmed.
They need to address the Void Ark loot pool and lockout, mostly. As it stands Void Ark is something you do once a week because you get a guaranteed upgrade item, but any more than that requires a battle against loot RNG and there's no reason to deal with Void Ark's mechanicsy wipe-prone insanity and lockout for RNG if you can just beat your face against a keyboard in Diadem for a while until loot falls out. RNG both ways, but you're less likely to leave Diadem empty-handed and if you can form an 8 man party you'll be fine in Diadem while I don't know that one good party can carry a fail VA alliance.
Like, I'm fine with 210s in Diadem, but you can't simultaneously have that and pretend i200 is special and needs to be protected.