Can you replace the spare TV with a cheap tablet that runs iPlayer or streaming channel of choice, freeing up your chest of drawers? If you transfer windows license from old PC the savings would buy a kindle
Can the PC overhang the unit at the front? Overhanging would be fine, even if it was a little unsightly.
The power increase over the past 10 years is absolutely huge. The wattage that performance parts put out is very significant and with the way auto boosting works to increase the clocks it makes the hardware very temperature sensitive.
The nano systems can physically fit the hardware, it's just really not a good idea. It's like years ago when people were putting 2l turbo engines into novas. They went like stink but you couldn't actually use them for anything performance related as they would overheat. It's possible to physically do but there are always compromises.
If there weren't...... larger cases that focus on airflow wouldn't exist. You should see the pain people go through with performance laptops.