I think there's a few things here where you may be confused.
Horizontal CPU heatsinks like the Noctua NH-L12 push hot air over the CPU.
The air going into the CPU cooler is not hot. What has it already been used to cool? In most systems if the airflow is coming through the front it might go over a disk drive, then over the RAM and some motherboard components but the heat it will pick up from them is trivial so the air is still relatively cool. Look at the motherboard temperature sensors and you'll generally see some pretty low temps that show this. So long as the air going through the CPU heatsink is cooler than the CPU is, the CPU will be cooled so this setup works well.
In most tower cases, GPU fans pushes hot air downwards, while hot hair physically goes upwards.
Most GPU coolers do not blow air downwards. Why did you think that? They generally push air upwards and backwards/sideways through the GPU heatsink. With designs that expel hot air out of the back this works well for overall system cooling. The fact hot air rises makes little difference as chimney effect only really makes a big difference when you have tall chimneys over which there can be a big temperature gradient.
Most current cases like the Fractal Design Defini Mini have the PSU under the GPU, so the GPU pushes hot air toward the PSU.
Leading on from my previous point, this won't happen as the GPU does not blow hot air downwards.
Most current cases have front fans that pushes air towards the back of the case, but the PSU is in the back and pushes air towards the front.
What PSUs have you seen that pull air in the back? Every PSU I know with a fan pushes/pulls air through it and out of the back.