On some level, you could make the argument no advancements have been made since the Girandoni carried by Lewis and Clark in 1803, in terms of the fundamental operating principles of a PCP. Just depends on what each individual sees as significant.
Materials science, for example, has advanced leaps and bounds in 200 years. Narrowing it to the last 10 or 20 years, the available operating pressures have shifted up and the vessels have become more affordable. That seems to me significant in the capabilities of big bores and also the viability of regulated “medium” bores like .30 cal.
Most of the advances as I see them are in the realm of better part precision and the ever faster adoption of yesterday’s premium features like regulators and repeaters into the low price segment. And the support equipment has absolutely plummeted, both hand pumps and compressors.
But to your point, I don’t see anything revolutionary possible in a system where air is the propellant other than the obvious of increasing pressure. Making any vast increases in efficiency seems unlikely because the mass of the air itself is significant compared to the projectile.
Meanwhile I want to recognize the closer you are to a technology, the closer you follow it, the less significant those advancements will appear. It’s easier to pinpoint them if you turn your back for 10 years.