Didn't DQ11 become the best selling title in the series? If is that so, I think there's a legit argument to be made in favor of keeping Luminary as default for recognizability alone.
I think DQ3 does beat it in current numbers(not finding an easy source), but DQ3 only sold one million in opening, while XI sold a whopping two million, already beating it out.
By 2021, XIS etc. did get up to 6 million worldwide. I'm pretty sure DQ3 has more overall sales, though?? However, it's worth noting one has a massive impact even then. This might also be part of why Luminary was chosen in the first place. It's the best selling game in its first opening. It was obvious why he was the main face, due to a remake being planned.
They also decided to base the content mostly around Luminary with the rest playing a clear second fiddle. Exceptions were a single trailer, and eventually changing it so Dragon Quest III is the year being represented for All-Star Mode(which mind you, doesn't actually make much sense. Ignoring bias, you should be using the first game in the franchise, where it all started, or stick with Luminary, the actual face of the Hero in Smash).
(Note that I'd prefer it being shuffled among alts so Hero can appear in four possible orders within All-Star, making each Hero feel like they're contributing something to it. They're all valuable and worth noting).
That said, there's also the common misconception that Kirby uses Erdrick's Equipment. He uses Erwin's Equipment, which is intentionally similar to Erdrick's(for many reasons). You could say it's the best of both worlds, since it still reminds one of Erdrick.
It's probably going to be whoever the newest Hero is (assuming feasibility) for the same reason it was Luminary.
But I'm pretty sure Sakurai wanted it to be Erdrick.
He confirmed a while back Erdrick was his personal choice. You can see a little clear bias in All-Star's change too. Albeit, him appearing in another trailer could be related to a thematic factor, not bias.
However, changing it also means they need a new amiibo, a better stage, new spirits, yet another change to All-Star(unless it gets another overhaul anyway), new model stuff and animations, possibly other major changes. It's probably more likely the new Hero might help them boost the amount of costumes from 4 to 8 unique ones, though, if he's even... similar to the proper proportions/abilities enough. I wouldn't put too much stuck into this case being a rotation. It's a lot more work than others. It's not likely that feasible.
Ah, right, I forgot to mention; as a 3rd party set of characters, they all are pretty costly as is. Rotating out means spending an extreme amount of money to justify every new change and amiibo alone. This is probably why we only got one amiibo in the first place. This isn't like Link who can be changed out on a whim since Miyamoto/Nintendo/etc. would at most just want to make sure it's a design they approve of. Sakurai isn't paying extra for every new costume here. Even a fifth costume is not going to be remotely cheap. Most 3rd party characters also tend to have weaker costumes, but it's not just "oh, hey, companies don't want the design to deviate that much", it still costs extra money for each design, especially if they're significant alts instead of clearly a new pair of clothes. Square-Enix along with its other companies that control Dragon Quest is quite a few, and they all get a piece of the licensing pie. That means even 4 costumes was going to be pricey alone. Changing out the main amiibo and face is not an idea that would go well. I mean, every new Link still looks like the old one. And considering, as noted, they can't get an amiibo for all, it's even harder to justify rotation here. Price just doesn't add up for anything beyond hopefully a new Alt or a Mii Costume, really. Or a Spirit. Whoever the 12th Hero is, there's no way they'll be absent if Hero in general returns, it's just not feasible for a rotating face. If they made amiibo for all of them, they could justify it on a whim, though. Nothing really changes(and they don't necessarily need to change the stage to fit, but it would make sense for each new Smash game, to get more unique content in and all).
