Could a balanced ecosystem contain producers and consumers but no decomposers why Or why not?

No. Things are produced from raw materials, and consumed by others. But the "things" - whatever they are, from leaves to airplanes - don't live or last forever. Something or someone needs to convert the dead or useless "things" back into reusable materials.

Trees and other plants grow, consuming elements from the soil, water from the rain, and carbon dioxide from the air. They produce leaves, fruit, and seeds. Animals eat the leaves and fruit (and other animals will eat THESE animals), and also consume water. The digestive systems of the various animals have their own "decomposers", converting food and water into feces and urine. Bacteria and scavenger animals convert the feces, urine, and the bodies of dead animals back into nutrients for trees and plants.