Planes work quite differently in this setting.

There are no discreet planes where outsiders live in anything resembling a "world".

Instead, scholars envision the Universe as a cosmic "sandwich", with the "upper" plane, known as the Aether, flanking the Prime Material plane along with its dark twin, the Nether.

This planar model underpins the entirety of known physics. Aether is the purest form of being, superior to physical matter and energy. Aether "devolves" into one or the other, never to return to the form of Aether through natural means. Thereafter, it acts as physics decrees, within the Material Plane. Inevitably, in cosmic time, all matter and energy decay into Nether, never again to return to material form, and contribute to the "plane" of Nether.

All things elemental, sylvan, magical, etc, exist within the Material Plane. If there are pointy-eared people, they are part of the Material Plane. Undead? Same. Demons? Well...hold on. There are true Outsiders, but they have a lot less in common with D&D's "demons" and "devils" and much more with something Lovecraft would dream up. Their existence is not compatible with the physical universe, and every moment they occupy it, they permanently damage its fabric.

For eons, natural energies were harnessed by the denizens of the world. Sylvans wielded power over life, animals, plants, weather, etc, as easily as humans wield spears and slings. Elementals, naturally, wield considerable power within their element, though they need to return to their power source to "recharge" from time to time.

Humans, well...perhaps it was their lack of innate magic that fostered their ingenuity. Necessity is the mother of invention, after all. If every rock, every tiny animal, every scrap of plant matter, all has some magical energy, well, it was perhaps inevitable that humans would exploit that energy, and create magical tools from them. Artifice, in this world, is near as old as Magic, and they have developed in tandem for ages.

In the end, Artifice proved far easier to scale and industrialize, and the scales tipped wildly as human civilization exploded in population and capability. What was once a vast world of natural wonder was quickly brought to heel by a handful of powerful human nations, taming the waves and the sky with their inventions, bringing the light of "civilization" to every corner of the world, and inviting the more primitive folk to resettle in their grand cities in order to "enlighten" themselves.

If the world has some grand, karmic realignment in mind, it hasn't shown any sign. Various rebellions and resistance movements have been tried, a few have seen brief success. But in general, a few thousand Sylvans, even armed with ancient magicks and protected sites of power, did not historically fare well against fleets of iron airships and armies bearing rifles and cannon.

If anything, the world seems to yield its power too easily, and the nations, quickly running out of new parcels to add to their imperial holdings, are eyeballing one another more and more jealously. And while the "unseemly" folk are often treated as second-class citizens--at best--their natural magic would seem a useful edge in future conflicts. Those who escaped exploitation in their now-conquered homelands may well be swept up in the rising arms races between empires.

The industrial revolution was enabled first by the simple mechanization and automation of ancient Artifice, but more importantly and impactfully by the discovery of Aether in its pure form, or rather reliable means of producing such energy. Where an engine using a fire elemental core would fatigue from heat or just burn out after modest use, pure Aether can drive the same machine with ten times the power and seemingly limitless endurance.

Despite its relative rarity, pure Aether is increasingly sought after for even the most menial uses. Gone is the unpleasant sulfuric smell of fire-core-driven street lights, gone is the buildup of salt in a water-core-driven washing machine; no longer do fine folk need endure unsightly weeds, buzzing flies, and parasitic pests in their gardens--nor even water and tend them, as Aether provides all the energy they could ever need. Amongst the richest families, it is even becoming fashionable to procure clothes that were never plants or animals, eat food that was never grown, drink water that never rained--all to avoid the filth and stigma of the world they pollute.

And yes, they do pollute it. Naturally-occurring elements do decay into Nether, theoretically, but they do so in cosmic time, enjoying virtually endless renewal over the comparitive blinks of an eye that life has existed in the world. But elements derives from the newly-drawn pure Aether degrades much, much more quickly. And while all Nether drains "harmlessly" into its designated plane, it is becoming increasingly apparent that there is a consequence for disrupting the natural flow so profoundly.

Theories abound, and everyone disagrees. The most superstitous blame every unlucky event on the abundance of Nether, or even the general souring mood of modern society, and the increasingly blase attitude toward violence that seems to be leading the whole world to total war. Others worry that the imbalance is affecting natural magic--not just that used by Sylvans and Elementals, but the very lifeblood of the world. And what little proper science that is being done seems to bear out the idea of an increase in Monster activity, as well as an intensification in the activity of dark magicians, going beyond the expected curve from increased population. And, naturally, everyone predicts this or that apocalypse as an inevitable result.