Consider the coil materials. Copper windings, copper insulation coating, steel core laminations, and polymer encapsulation.
Copper failure modes: corrosion when exposed to elements (encapsulation shields from this), melting from heat (overcurrent or external fire), high speed projectiles.
Steel core failure: same as above.
Copper wire insulation: this is a coating on the wires to keep them from contacting each other when layered. Damaged by heat (overcurrent or external fire), weathering ( when exposed) , maybe vibration, high speed projectiles.
Coil encapsulation failure: weathering, polymer breakdown due to age, Heat (overcurrent or external fire), high speed projectiles.
So, if the outside encapsulation is intact, the internals are mostly protected for a very long time and function the same as when new if not abused in some way.
I’ve not seen Honda’s choice of encapsulation material to fail yet with normal use. But, they are not hatchet proof.
The ignition leads are another matter. The aluminum wire stranded cores are very robust. But, the outer insulation material hardens with age and cracks when forcibly bent. As they are molded into the coils, the assembly is rendered useless when the leads crack. In place, they don’t need to bend, and function fine. But, servicing the plugs places them at risk. If they don’t crack too close to the main body of the coil, there are splicers to replace most of the ignition leads. There are reports that the coils can be dissected to replace the ignition leads. It’s not an easy process and success in not assured.
Aftermarket coils can certainly work. But, the materials and design chosen may or may not last the 50 years plus that Honda’s coils have proven to work.
As long as the motor remains in stock configuration, the stock coils work as they did new. If you increase compression, or spark gap, only then might you expose the need to have coils and ignition parts rated for higher voltage output.
