Before I begin, just note I haven't played Pokopia (I have no interest in playing it so don't mind spoilers) but I have been keeping up with playthroughs and story revelations. So I may well be missing some context that those who played the game may be aware of. Still, I feel I know enough to have formed some theories, or at least thoughts, to add in my two cents.
First, when it comes to the timescale, I wouldn't be surprised if a
floating timescale was purposely done in order to give both a sense this game is happening in the "distant future" BUT not so distant that records from modern day (or ones which treated modern day as happening in a relatively "recent past") wouldn't still be around if scattered. The game's story feels like it was made around the game's concept: a world where there's only Pokemon who are building up/rebuilding the world. Thus they decided they needed an explanation what happened to all the humans and how the world ended up like this, and from there the game's goals and progression are planned. In a way you're not supposed to think about the timescale, which in a way makes sense being your perspective is that of a Pokemon which the game has gone out of its way to show take things at face value and present focused; they're intelligent (some more than others) but are also and foremost instinctual. This could be why, despite the suggestion of a century if not multiple has passed since humans left, Ditto thinks it'll see its trainer again: It wants to see its trainer again, that's all that matters and a pesky thing like time (and aging & lifespan) is something that can be thought about later if at all.
(BTW, I personally also take Pokopia as being a "What If/Elseworld" story: What if the Pokemon world was struck with global natural disasters that forced humanity to leave with the Pokemon left to rebuild. If not a cautionary tale, one could easily take the inhospitable global nautral disasters which are essentially terraforming the planet as analogues to Climate Change and how its effects on the environment is noth now being felt and worsening unless humanity cleans up its act. Infact the whole "humans escape to space planning to come back later yet haven't" feels very remiscnet of
a certain Pixar film...
and its plot twist)
As for Trangrowth's age, well let me ask you this: Do we know how the Tangela family age? Do we even know that much about their biology that they "age" as we think they do? When it comes to aging and reproducing, plants can be weird. Like, fun fact, did you know that generations of "visible" trees are one generation apart because the "parents" of visible trees are nothing more than spores living inside the offspring they eventually sprout? There are even some notable plants that don't even reproduce but rather clone themselves. One particulare example is the a quaking aspen clonal organism called "
Pando". Despite looking like an entire forest, its all one giant organism that's thousands of years old. Every single tree that makes up Pando is a clone of one another, all connected by a giant root system which continues to spring up more clones. Who's to say the Tangela family aren't a similar like organism? Their vines are constantly cloning and replacing themselves, and as long as they have enough nutrients & remain healthy to continue this cloning propegation, they can live to be thousands of years old. BTW, don't take Professor Tangrowth's color to mean its reaching it's "end", some plants are able to enter a form of dehydrated hibernation when the environement doesn't suit their need but spring back to life when they do, they're called "
resurrection plants"; Professor Tangrowth may look dried out but give him a few months and maybe he'll clone new healthy vines and get back some of his blue color.
Second, when it comes to mysterious crystals they're kind of a dime a dozen in the Pokemon World and GF loves to add them in, even if by retcon. And in this case, the most modern depiction of
Cerulean Cave in Let's Go has added a whole batch of these crystals. So it's not exactly surprising nor new that there are mysterious crystals in Kanto. Now, that's not to say they won't retcon a connection to Terapagos, them having added in those crystals before SV makes it even easier to do so. Plus they kept the lore behind Terapagos both so simple and vague it wouldn't be surprising at all if there aren't Terapagos all over the Pokemon World, hidden deep underground and not having released their primordial/reality shifting energy to create a big crater like the one in Paldea did.
