Piston cleanliness is not everything. It just shows how well the detergent in the oil is doing it's job. It is the surface mating in the engine that counts. Not by wearing the engine, but by rubbing without wearing. That is the mark of a good oil. That is why good synthetics hardly show wear.
Jaso FC class oils are excellent. The FD class added some extra hours to the grueling wear tests, and also added extra detergency (right word?), to clean out the carbon deposits that are formed in the normal combustion process. In other words, keep those particles in suspension until flushed away.
I wonder: "flushed away by what?" . By replenished oil at a 1:100 ratio? I.E., Burn one liter of fuel in half an hour, containing 10cc of oil. Most of this oil passes right through the engine without doing useful work. The oil particles that make it to the metal parts are few, and hardly capable of flushing anything. So they are engineered to cling to the surface like it's the last thing they need to do. The flushing takes place when starting with the choke on, and the engine is wetted with gas.
These carbon particles are extremely small, because they are not allowed to accumulate and bake together. (oil's job). Their size is so small, that the oil film is thicker. They thus cannot harm the engine.
The story is different with dust particles that float in the air, or are stirred up by the prop wash. These particles are thicker than the oil film, and so they start abrading the contact surfaces that should be burnished. Clean engine nevertheless, but wearing faster than it should be.

Now to return to air filters. These sometimes just cannot be accomodated in the plane, even though they at least double the engine's life expectancy. They also increase the engine's performance over time. The unfiltered air engine never realy runs in. It wears, and by doing so prevents proper running in.
As long as the engine survives we see no problem. Crash damage and "moving on to newer pastures" never let us wear out an engine. So air filtering gets no priority. It does not change engine useful life for our purposes. It does however provide cleaner planes and better running engines, until they are put on permanent store.