Survival depends on patronage. We have so many varieties of people with varying tastes and temperaments. People patronize different Gods to suit their tastes and preferences. So, naturally so many Gods are there. Different strokes for different folks. Gods have to stay because people keep worshiping them and people are there continuing with their Gods because they do get their blessings and succor through their prayers!
Win-win situation. That is the beauty of Hinduism’s “ishta-nishta” (steadfast devotion to one’s favorite God-form) concept.
In reality, God is one in Hinduism. But each devotee has been given the freedom of devoting himself to his ishta with a full conviction that his ishta is that one supreme God. Such a belief strengthens his devotion. That’s why Shiva puranam would say Shiva is the supreme God and all other Gods worship him. Devi Puramnam will say that Parashakti is the creator-protector-destroyersand Brahma-Vishu-Shiva are all subservient to her. Same with Vishnu, Muruga and so on.
People at lower rungs of religious practices tend to fight and argue with other believers as to who the real supreme God is. Let them. It is a matter of time before they mature and evolve. It might even take a few more births to grasp the unity in diversity, but it does not matter. Better to be a believer than an atheist!