Audi opened a hot air of 90 degrees, and the water temperature dropped to 70 degrees. What is the reason?

If the real water temperature is only 70 degrees, it is unfavorable to the service life of the engine, because the normal working temperature of the engine should be above 90 degrees, and too low temperature will affect the service life. It is estimated that the thermostat cannot change its opening with the change of temperature, so it is better to change a thermostat (core).