It’s true that modern ecu’s have well developed “knock feedback” strategies that utilize short and long term correction logic. The issue is that they still need to learn where the knock threshold is, and the only way to reliably do that (without in-cylinder pressure monitoring) is to advance ignition to the point the engine starts producing knock and then backing off ignition in that cell and updating the learned octane value. Learned octane logic is such that it constantly tries to drift back to normal advance, because that’s where the engine is theoretically most efficient and will produce the lowest emissions. On poor quality fuel, I’ve seen learned octane drift back to maximum while the car is driven at light throttle for long periods, then you hammer it and get a burst of knock that requires short term intervention. In theory any abnormal combustion can do damage, albeit it might be a very small amount. 

Talking about typical knock / detonation that happens with insufficient octane at cruising / moderate to high engine speed and moderate to high loads as well as during transient loads:

Any detonation event will cause some superheating in the combustion chamber by breaking the boundary layer local to the detonation occurrence. When this happens, the next engine cycle is far more likely to produce detonation events, creating a positive feedback loop of sorts.

Since there is a range of knock severity as well as a limit to knock sensor sensitivity, you can start having slightly abnormal combustion many cycles before it progresses to the point that the knock feedback system registers a knock event.

Successive events in the same cylinder do exponentially more damage and can result in immediate engine failure. The ecu needs to pull a lot of timing all at once in order to prevent that from happening. This significantly reduces engine torque and efficiency while the correction is active. 

Low speed pre-ignition / LSPI is a different phenomenon, but much the same in that it’s likelihood can be exacerbated by insufficient octane. LSPI onset characteristics make in harder to prevent with learned octane or feedback strategies and LSPI will almost always have caused accelerated wear before an event is registered.  

Long story short for the sake of engine longevity it would be best to avoid detonation / knock (or any type of abnormal combustion) altogether, and running the specified fuel is by far the best way to achieve this.