Fair point. A few things I do have a concern with, though.
In regards to the Battle Frontier in HGSS, you could've nodded your head and explained the phenomenon as the fact that it was just a newly built area in Johto at the time, and the same could be explained of the Pokeathlon and the same could be explained of Pal Park in Kanto, which didn't exist originally either. In regards to the PWT, it can easily be explained that the reason the characters appear as they have is because they just simply happen to travel there for battling purposes (also no one ages in Pokemon, really. Not even Cynthia between Platinum and B2W2!).That being said though, I feel there's a difference between a new area spontaneously appearing and somehow stumbling across a past region in its entirety.
Oh no, I meant the "weird" part of the Battle Frontier in HGSS is that the Frontier Brains are essentially in two places at once, Johto and Sinnoh, even though that's impossible, even by Pokémon standards. Not that it's weird for there to be a Frontier at all.
And yeah Cynthia didn't age, but she's an older character, I think. Children age visibly more than adults even with only a few years difference and I think that's why Caitlin looks older in BW/B2W2 when compared to DPP/HGSS, but a character like Cynthia doesn't.
I'm a person that likes consistency within reason, for the most part. For the features explained above, I feel there's an obvious reason behind it. For the Battle Frontier, it can easily be explained that Platinum had one, ergo it wouldn't really look good if HGSS, despite being a Gen II remake that did not originally have the Battle Frontier before, somehow mysteriously didn't have it. The Battle Frontier, the Pokeathlon, and Pal Park in Kanto are new additions and features that were added mostly for the sake of "modernizing" (so to speak) older regions and make them more expansive and exciting. If you just played through plain old Kanto and Johto barebones without any of the additions that Gen IV provided, while that would've in theory made sense for consistency for the most part and stayed roughly in theme with what the original games were about, I can imagine a large majority would be displeased at not being able to have the game feel more "modern" without the features and additions that Gen IV provided, blah blah, you get the point, really.
I absolutely get your point. [looks into the sunset as Hoenn's Battle Frontier fades from view]
Now, another region entirely? That's a harder pill to swallow, I'd think. Like say, for example, if you were to travel from Alola to Sinnoh randomly, with no explanation at all, even in post-game, would that not raise questions of how you were able to travel to Sinnoh in the first place, even though Sinnoh has nothing to do with Alola, at all? It's harder to come up with an explanation to randomly shoehorn a past region because it's not a new feature by definition, it's an area we've been to before, it's an area which has a completley different culture than the one you're visiting, a region which doesn't have any ties whatsoever or has anything to do with the region you originally came from... I mean, I'm sure some people would ignore the more logical aspects of this, but this is why I continue to be skeptical of us going back to past regions to this day.
But is it actually that difficult to shoehorn in a reason? Where are the sun and moon located? In space. Which region dealt with matter regarding space? Sinnoh! Sprinkle in a few lines regarding Team Skull's connection to Sinnoh (anything will do--maybe their leader went to college with Cyrus) and suddenly a lot more possible reasons for the two regions being connected can pop up. It really doesn't take much for two regions to have relations.
In short, past features returning is one thing. It's not hard to randomly fit in a small area in a current region and drop a Battle Facility that was once in the past there, look at ORAS for that matter and the Battle Resort! Past regions, on the other hand, you'd have to find some sort of a plausible explanation for. What would you even do in a past region? How much would you get to explore? That kinda thing, y'know?
As for how much you'd get to explore, I don't think that's much of an issue any more. It certainly was back when the series was still on GameBoy cartridges, but fitting two entire regions in one 3DS cartridge isn't all that tough, data-wise. What you'd do is the same thing you did in Kanto back in gen 2 and its remakes: travel around and get the badges! A minor plot similar to the Looker Episodes wouldn't hurt, either.