True, well argued.
For there to be an energy flow, the life needs to be in a temperature range intermediate between the source and the sink. I'm still a bit worried that if there is a sufficient energy source (most likely a star or geothermal) it will raise the entire environment of the planet significantly above the ambient universal 270K.
However, it really isn't a significant issue. If 270K cosmic temperature is too high for life on planets for whatever reason, it will be comfortable a few million years later. The basic argument of 'a few million years friendly for life everywhere' still holds.