Rather than suggest banning the cards, why not get creative to play your best decks against it? Maybe make a tweak or two to fit the biggest competition in the format and learn how your deck can work against new things. I've been playing a Dusknoir X deck (based on in-between damage with Spirit Pulse) and even when new cards continued coming out, I handled the changes just fine against them. It's not necessarily that the cards are too powerful, but that some players don't want to have to adapt to an ever-changing environment.
As for Gengar Prime, it's not exactly unbeatable. There's a fatal flaw in the Gengar Prime attack: it relies on the assumption that its opponent actually has any Pokemon in their hand. If players act carefully against Gengar Prime to minimize the time they hold onto Pokemon cards, just like players had to minimize trainers in their hand against Gengar SF, they'll be fine if they just play smart.
In the case of erratas, how many erratas get issued only on a card's playability? Errata's I've seen had to do more with a slight mistranslation from the original Japanese text. That was also the reason for the old Slowking ban, As I recall. It was supposed to have its body in effect while active, with a junk heap of an attack, and at that time it was run by Wizards of the Coast who preferred not to errata their cards.