It is nowhere near as simple as decreasing temperature with altitude. (watertechbyrie.com)
Your argument if quite the flowers that bloom in the spring have nothing to do with the case, just as the temperature in the thermosphere and ionosphere have nothing to do with the greenhouse effect but the decline of temperature with altitude in the troposphere is determining. (realclimate.org)
«As we mentioned earlier, in our explanation of the greenhouse effect, OLR reduces (with an increase in C02) * because of the decrease in temperature with altitude *. (planet3.org)