I can't much speak for regional forms regarding the Watsonian way of looking at things, but I would presume it would depend if the regional form serves a niche or not.
When approaching such a topic, I think it would best to consider things from a game design perspective as to what niche regional forms serve. As they are are at the present date of writing this, regional forms can serve as an alternative to creating a new Pokemon from scratch, using existing assets and prior lore created about them. This helps reduce the logistical burden of crafting new regions and thinking up new mons for them, as well as giving their already existing audience something familiar to draw from prior generations.
That, and it allows Gamefreak to update existing designs to either supplement or outright replace ones it may deem to not fit the current status quo, especially with the unwieldy number of mons it has to manage as time goes by.
A rattata is a rattata. But place it elsewhere, draw upon IRL comparisons in certain regions of the world, and volia, you have your regional form. Add a bit of eccentric design shifts to accommodate modern Pokemon design philosophy, and you have something you can ship out with relative ease without the headache of worrying about new mons to deal with.
While I cannot say I am satisfied with the execution of regional forms regarding some of the design elements influenced by modern Pokemon design sensibilities, I cannot deny it works for the vast majority of Pokemon's intended audience, out there as the may be. Not to mention, the "plushification" process for the lack of a better term, seems to endear most people to the aesthetic. One need look no further than to typhlosion's newer iteration. While anecdotal, it would appear the popular consensus is that it is superior to its Johto iteration. It's tamer than I would expect for what I would expect of a redesign to entail in the modern era, but it clues in to the possibility the same can be done for other mons from past regions, be it to give them a new "lease on life" or just make it easier from a production standpoint to manufacture their merchandise.
As for retrofitting the lore from prior regions to "force" new regional variants into existence, it's nothing that a retcon or two can't solve. Such a reality shift can even be justified in universe through the Sinnoh pantheon, or even that from Aloha's wormhole stuff. Again, it is not something I would personally be fond of, but I doubt most people would take too much issue if given a chance to dive into a new reboot of past regions, with new looks to old mons on top of that. You can even tweak such mons to have a much stronger focus on anthropomorphism and theming to keep in line with the newer trends that starter mons and the like have of late.
A possible, albeit extreme example of what that may look like.
While it may not keep to all the design conventions modern starters may incorporate as of late, I could see something along this progression being the logical conclusion with regards to the focus on theming if you were to approach Johto as if were being created for the first time in 2024. Again, I am not personally a fan, but it works as a rough template for what a Johto starter reboot could look like as regional forms if given the opportunity.
Tl'dr, making regional forms is cheaper and eases off the logistical burden of monster creation.
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Expanding upon the lore perspective of how it such forms would come into existence, just steal how the process is done IRL, and tweak it to fit Pokemon standard.
New region? Borrow from eevee and explain it away that the environment, local spiritual/evolutionary energies influence the mons coming into the region, and vice versa. Throw in an explanation that each region has some form of "spirit" or "soul" that redefines whatever mons come into it.
Or heck, even branch into alternate evolutionary concepts like type shifting, perhaps having a mon shift their type on the fly with some special man made item, or have their forms be influenced by the human psyche and imagination to further influence their design path. It would be a partial explanation, if not outright justify the shift towards mons in theming and more... Out there designs such as sword and shield bearing legends of Galar, the focus on spys, soccer/football, and music from their starter trio, or even that one coal cart as a partial explanation, perhaps a manifestation of human spirit of some form.
Then again, might not be doable with regards to the logistics behind that, creating complexity where none should exist. But it would allow for the "base" form of certain mons to exist in the wild in a more "naturalistic" capacity like that of the earlier generations, while allowing for the more exotic shifts seen in newer mons. Say, scorbunny hailing from some region in some version of the Scottish highlands as a fire/rock type, kicking boulders and what not towards its natural enemies, and slowly shifitng into a new "breed" of mon as seen in the Galar starter towards a focus on socceer theming with the afformentioned human influences on its psyche connecting it to humanity.
On that note, I would even go so far as finally allowing the ability for you to "talk" to your Pokemon, or vice versa. It *might* be controversial, but we already have some precedent in the form of Meowth and some telepaths. Not to mention that translator device mentioned back in Ruby/Sapphire/Emerald with the Devon Corporation. I can't say as to what the proper limit would be for that, but you might as well poach that from Digimon and incorporate it in some form to be gained as both human and mon alike bond together through the power of friendship as a theme. It would fit nicely as per the first English Pokemon theme song of "You'll teach me, and I'll teach you".
From a gameplay mechanic standpoint, it would allow for new potential for growth and bonding with your buds, and enhance it by just asking what certain mons may need, open up potential questlines involved with them, and perhaps Pokemon take a further place as some form of "equal" of sorts. If not in the mainline game, then I could see something to that effect working in some side game as a testing ground, like Legends Arceus.
