They can repurpose Megas as Gigas with the explination that Gigas are the true form of megas. I don't see them bringing back Megas or Z-Moves at all in the future, nor as a postgame feature for competitive, so they have to do SOMETHING with them. For example the moves changing to something else depending on type and if its attack or status is already Z-Moves in all but name, so why not megas becoming Giga Forms? Mega Gardevoir for example becomes G-Gardevoir, Mega Pidgeot G-Pidgeot and so on so forth.
I mean, they can do whatever they want at this point, but I still don't think they would use an existing form, which is even trademarked, interchangeably with a new concept. Especially since megas will still exist in Masters and other media, Pokken, Super Smash, probably the TCG and so on.
This is not like Clefairy or Snubbull changing types forever, this is simply a case where Mega evolutions won't be possible in SwSh, but it doesn't mean they're forever gone. So they can't just repurpose those designs for other things. It's as if they said that now Mewtwo will be the Gigantamax form of Mew, or that Rhyperior will be the Gigantamax form of Rhydon so they can still incorporate its design without adding an extra Pokémon. Or if they suddenly said Mega Mawile is a new Pokémon, a proper evolution of Mawile (I wouldn't mind, but that would be just weird). Things, especially Pokémon and their alternative forms, I don't think they can be recycled like that. Especially for that consistency they like to keep as Serebii explained a few pages ago.