.... Look at the various pagan religions from our world. Despite how they have been written up in all the editions of D&D, how many of them would even be truly Evil? A lot of them have grey areas to what they are patrons of and would only be evil, or even more Neutral with evil tendencies.
There you picked an interesting topic, the major monotheistic real world religions do have the same Lucifer / Satan incarnation of evil. But if you get further into the lore you find different demons/devils. They are usually fallen angels in eternal punishment, so they never outrank God or are equal in power.
The situation in fantasy realms is more complicated. In several official pantheons the highest creator god is (true) neutral and does never interfere directly. His subordinates are of equal power some are good some are evil some are neutral.
If you were to install e.g. one big bad overlord versus one good overlord instead, then you get a problem if players get enough power to confront the big baddie: If they win they destroy the balance as well as their biggest challenge, ending the campaign.