You are worrying here if your machine will die from thermal stress in 100 or in just 75 years really.
Sure there’s always the chance that a part will die to heat, but a computer is a tool. Tools wear off if you use them.
And they are one of the few things still built to last, simply because they got a short life cycle due to a lack of computational power after a relatively short period of time, creating returning customers. If the power would be enough for a decade I am sure CPUs/GPUs would have planned obsolescence…
As long as the T-max of your components stay below the manufacturer specifications it should be no problem to run 24/7.
My machines often run for weeks without a pause with 100% load and there’re no issues besides a room with warm, dry air.
Continuous thermal stress is less of an issue than repeating thermal stress in short bursts - obviously the expansion and retraction from heating cooling can cause micro fractures. But that’s really more of an issue with Notebooks and other mobile devices. In my past 25 years I haven’t really encountered any case of a regular graybox failing due to thermal stress.
The one rule to obey: Make sure you got a good airflow in your case and expel unwanted heat.
Every cooling system can only cool down a delta temperature relative to the surrounding air.
Easy example: Component 70°C, Air in case 50°C, Delta power of cooler 40°C
Obviously you could cool your component to 70-40=30°C.
It’s impossible though because you can’t cool below the temperature the surrounding air has. 70-x=50 -> x=20°C basically leaving you with wasting half the possible cooling power - and to make matters worse, the air inside the case will continue to heat up, especially if you got a decent metal case and it starts to heat up as well.
Also keep in mind when cooling the case that you can either have a high pressure or low pressure air cooling.
For low pressure you have more air volume expelled than shoveled in.
For high pressure you have more air volume intake than you expel.
Relevant? Yes. High pressure is better. If your fans (with dust filters) shove in more air than you expel you get a high pressure system in the machine and all the gaps and holes in your case blow air keeping dust out of the machine.
If you expel more air than you intake all those gaps start to suck in dust.
And dust is a very good thermal isolation preventing proper heat transmission for cooling.
Modern fans are labeled in m³/h so it’s easy to find out which system you got.