As the mulch decomposes, new plant roots will grow into the shrinking layer. Mulch should be too moist and too active for colonies of ants and termites to form in. Mulch is a soil amendment. A mostly carbohydrate material becomes mostly protein via the DNA of the soil flora and fauna. Think of it as micro-range management.
Cows eat carbohydrates and make protein in a similar manner.
The soil organisms draw the material into the soil profile as they reproduce, providing better ecologies for one another. Organic matter additions must repeatedly be added to the soil just as gasoline must repeatedly be added to cars. The material will be oxidized, respired into the atmosphere and work will be done in the soil. Soil structure will result. To maintain those openings, if you use the soil, harvest plants, walk on it, etc.