If the universe ends, is there really any point in our struggling to survive?

You are programmed to survive. It's an instinct that even single cells have - to find food and energy, so that they survive. No religious entities required. That's how living things work. The universe is NOT going to experience heat death for billions to trillions of years from now. IF you have no motivation to survive and have children to transmit your genes to, that's your problem. Just be happy that you can sire children. Some of us can't have children, for various reasons, so our genetic information is removed from the gene pool when we die. That's why it's called "survival of the fittest."