It is true that older fixtures would dissipate more heat but that was only a byproduct of the fact that they were designed to operate at higher overall temperatures to begin with. Most incandescent fixture were required to be able to withstand the heat output of a 100w bulb without melting, catching fire etc. What this means for an LED in a fixture designed for an incandescent is that it is likely to consistently be operating at high temperatures even though the housing might feel cooler than with an incandescent. LEDs just don’t have the same operating range as incandescent bulbs and will fail prematurely if you treat them the same