Note: The question's author says that 5e is preferred but that explanations from any edition are welcome.
In AD&D spell energy comes from other planes
The Dungeon Master's Guide (1979) on Spell Casting says
All magic and cleric spells are similar in that the word sounds, when combined into whatever patterns are applicable, are charged with energy from the Positive or Negative Material Plane. When uttered, these sounds cause the release of this energy, which in turn triggers a set reaction. The release of the energy contained in these words is what causes the spell to be forgotten or the writing to disappear from the surface upon which it is written.
The triggering action draws power from some plane of the multiverse. Whether the spell is an abjuration conjuration, alteration, enchantment, or whatever, there is a flow of energy - first from the spell caster, then from some plane to the area magicked or enspelled by the caster. The energy flow is not from the caster per se, it is from the utterance of the sounds, each of which is charged with energy which is loosed when the proper formula and/or ritual is completed with their utterance. This power then taps the desired plane (whether or not the spell user has any idea of what or where it is) to cause the spell to function. It is much like plugging in o heater; the electrical outlet does not hold all of the electrical energy to cause the heater to function, but the wires leading from it, ultimately to the power station, bring the electricity to the desired location. (40)
In short, the gods put the knowledge of how to cast spells into the minds of their clerics et al. so those dudes don't have to study but pray. However, magic-users et al. must study their spellbooks to memorize their spells formulas. Nonetheless, both end up just casting spells that draw from the same source: the Negative Material Plane and the Positive Material Plane.
So using this source-of-magic idea it's kind of impossible to say that arcane magic is fading unless the DM's going to rule that both arcane and divine magic is fading. And if using this explanation as the campaign's source-of-magic idea and divine magic is not fading and arcane magic is, then, I guess, there's the plot. (Hint: Jhkdad the Unfathomable is draining the energy from both planes and using it build his own universe—it's only a matter of time before the gods expend the last of their power and divine magic starts fading, too! Better hurry, heroes!)
However, were this DM to attempt to concoct a plot around fading arcane magic in a setting using this source-of-magic idea, this DM would make the plot much more conspiratorial: clerics have been sabotaging wizards' spells for a long time—a stray symbol here, an extra syllable there—so as to weaken arcane magic deliberately. Ancient stories talk of apprentice wizards casting prismatic sprays but, over time and through subtle manipulation these clerics have turned what was once the low-level spell prismatic spray into what modern wizards recognize as color spray and made the spell prismatic spray hideously complex… far more complex than it actually needs to be! But, recently, the clerics—for whatever reason—have moved too quickly and too boldly and even apprentices recognize their spells aren't as powerful as they remember their mentors' spells being. How did these clerics manage to alter the spellbooks of some of the settings' most powerful wizards? Why would they? The heroes are in a race against time—soon, the only wizard spell anyone can master will be read magic… and that'll be a 9th-level spell.
