Voadam
Legend
In the 5e DMG, under "Other Religious Systems," it says: "Monotheistic religions revere only one deity, and in some cases, deny the existence of any other deity. If you introduce a monotheistic religion into your campaign, you need to decide whether other gods exist. Even if they don't, other religions can exist side by side with the monotheistic religion. If these religions have clerics with spellcasting ability, their spells might be powered by the one true deity, by lesser spirits who aren't deities (possibly including powerful aberrations, celestials, fey, fiends, or elementals), or simply by their faith." (emphasis mine)
The DMG specifically states that clerics (specifically clerics, not "priests" or "worshipers" or "warlocks") can gain spells from fiends.
That seems like it can reasonably be read as an if-then situation and not a generally applicable rule. If you have a setup of only one god and other religions with spellcasting clerics, then their spells might be powered by fiends. It seems like it can be read that one option in a setup with no evil gods is to have fiends change from the default to step into the normal role of evil gods.