I see where you're coming from, but on same note, I think that this is a good way for them to actually release "current gen" HA legendaries without requiring a ridicolous amount of limited-time events for when Natdex and GS cup VGC laps start.
If you think of it, we havent had VGC-legal HA legendaries for.... 3 gens? I think the only legal HA legendary in gen 7 was Lugia from that Japanese only event... and .... that's it. I don't remind if gen 6 had any legal HA legendary at all, but correct me if I am wrong there.
They'd still be pretty difficult to obtain (maintaining the rarity of HAs) considering you have to go through whatever that co-op system is AND highroll the HA %, combined with likely needing to "fix" nature and IVs of the legendary you caught, and with ridicolously low % of a shiny (though, they could even be shinylocked for what we know) but at least they would be available.
I am really suspecting either a galarian form, a dedicated Gigamax, or both.
Expecially the fact that the Regis were the *only* legendary so far shown outside of a Dynamax Raid or trainer battle is giving me a huge vibe that collecting all 5 regis will lead to this new Regigigas ... expecially because if they don't, why bother keeping it as the only unconfirmed legendary so far.
I know both my statements can be countered with "GF and logic in same sentence?", but hey, let me hope there ok? :P
edit: By the way, there's something I wonder about Urshifu: we know he'll be allowed in VGC, what I do wonder is, will it count as regular legendary, so be allowed right away like Tapus for example, OR will it account as restricted so only in GS cup?
Because one thing is to have that protect-ignoring monster around with regular pokemon, another one is to have it when the powerlevel is significantly higher.