Only Leafeon and Glaceon "evolve adapting to their environment" (on a side-note, Eevee-->Glaceon is amazingly close to Alola forms of Sandshrew/Vulpix in that light), the first 3 are purely evolution from being exposed to a strong elemental source for example. And Sylveon clearly has nothing to do with environment, with it being more akin to Espeon/Umbreon from a strong bond with the trainer, but without exposure to daylight or absence thereof playing a role. And I'd even argue that it's a lie for LEafeon and Glaceon, as it's not just "the environment", but very specific rocks reminiscent of the evolutionary stones in the first place which cause said evolutions.
But I completely agree. Let's stop pushing eeveelutions into new twists that their concept was never about, they've already been taken further than intended just for the sake of fanservice.
Eevee was always complicated because they always made exceptions for them, and were restricted by game mechanics; first, it was the only pokemon to have a branched line (hence why it used stones instead of natural evolution; in the days of RBY, environmental evolution was unthinkable). Eevee is the only pokemon that can evolve with more than one elemental stone; an exception to the rule.
Then, they had to add two more evolutions,
without stones, so they did that "friendship at day/night" thing. Granted, they did expand this evolution form to other pokemon later, even if at the time it was the exception.
After that, they still couldn't make environmental evolution (I imagine, because they might not have wanted to be forced to include all the necessary climates in subsequent games), so they improvised a couple of items that would work like evolutionary stones, without adding two new evolutionary stones; eevee is the only pokemon that uses them; another exception for eevee.
And finally, they had to add yet
another evolution, but couldn't come up with anything (sylveon isn't, unlike the rest, linked to the environment as element, just to the environment as "having a caring trainer", which isn't necessarily exclusive to it, really), so they had to improvise yet another friendship-based form of evolution; I understand sylveon is the only one who uses pokemon Amie, right? Yet another exception.
The point being, trying to find a consistent logic behind the in-game adaptations of eevee is, at the very least, a quixotic endeavor, because there is no logic, just needs and patch solutions (exceptions), because eevee is one of the most marketable pokemon and the eeveelutions are merchandising gold; as you said, they already pushed it too much because of fanservice; I do hope they end before they go too far.
Anyway, because of that, I'd much rather go by the anime's rationale of eevee having an unstable DNA that makes it prone to evolving spontaneously, and the resulting pokemon be perfectly adapted to the environment (or time of day, or trainer) where it evolved.
