"since it appears trying to actually do anything with it will kill it in any real sense."
That depends on the processor, existence or size of heat sink, heat transfer connectivity between SoC and heat sink, ambient temperature and OS settings.
Most modern SoCs have a temperature setting inside that the OS can read. If the processor gets too hot the CPU can throttle back the clock and reduce the heat produced. It could also schedule fewer or less powerful cores. Of course this slows down your computer.
The other main consideration is ambient temperature. If the air around the system is really warm the heat sink can only exchange so much heat. The fan helps with this, but in the end you might still have the CPU throttled.
This explains why some over locked machines used by gamers even use liquid cooling to carry off the heat from the circuitry.
I have not looked into the RPi 5 enough to know the extent of the thermal protection it enjoys, but I would not discourage you from buying the fan to get maximum performance on benchmarks.
There was a good "Toms Hardware" video on this with regards to the RPi 5
when it first came out.