This thread seems to be going all over the place.
First, while related, there is a distinct
difference between porosity and permeation.
And again, it is easy and inexpensive to make plastics non-porous and non-permeable. As noted above,
Remember, very caustic stuff like bleach, ammonia, even lye drain opener is stored for years and years in inexpensive plastic containers. Those liquids don't leak, evaporate, "eat" or permeate through the plastic. The casings of car batteries are made of plastic.
Look at our homes and the use of PVC plastic pipes. Why is it used? Because it is cheap to make, easy to work with and very safe and effective. Our clean water supply pipes are even under pressure - up to 80psi - many more times higher than in these coolers. If those water pipes were porous or permeable, they could not be used.
Drain/sewer pipes are commonly made of PVC too and while not under pressure, if they allowed that "waste" escape, it could be an environmental and health hazard to humans and wildlife.
On the other hand, some plastics are designed to allow significant and specific amounts of permeability. For example, IV (intravenous) lines contain plastic membranes specifically designed to allow the liquid medicine to pass through while blocking air bubbles.
In our grocery store, some plastic bags are designed to allow fresh produce to "breath" though the bags.
Permeation is at the molecular level where smaller
individual atoms and molecules are maneuvering past larger molecules.
There are over 5 sextillion (5 x 10^21 or 5,000,000,000,000,000,000,000) atoms in a single drop of water. If you see or feel a drop of coolant you have a leak.
One thing we have not really discussed here is "time". And sadly, the OP did not mention how old this cooler in question is. And I also have been remiss for not factoring in time.

My apologies for that.
As I said in my first reply in this thread, "permeation certainly exists". But I still have my doubts permeation is the big problem in CPU coolers as some believe. HOWEVER, with time being a factor, I can certainly see where a sealed, liquid cooling system could lose enough coolant due to permeation to affect performance. But with a "quality" cooler from a reputable maker, that would be years. How many years depends on too many variables to be specific. But, IMO, 5 - 6 years should be expected.
But then why do they fail at that point? Is it due to too much air in the system reducing the heat extraction capability to unacceptable levels? Or because too much air in the system prevents the lubricants in the coolant from properly lubricating the impellor motor? I'm leaning towards the latter.
Maybe it is time to go back to the days when I and many of the other old-timers here started experimenting with alternative cooling solutions - BEFORE there were AiO CLC systems. They weren't closed systems. They weren't even factory made systems. They were home-built with parts from fish aquarium/pet and hardware stores. That meant they could leak. Air could get in. Mold and other nasties could start growing in there.

But it also meant we could bleed out the air and "top off" the coolant levels.

Those were the days.