There are still many other things in certificated aircraft that are changing regularly based on ADs. This means “we found something in this decades old design that shows up when the fleet is in its 30s.”
Aircraft have VERY long lifetimes in comparison to most things. I own a 1962 Cessna - it is on its second engine, the first having been overhauled once. Some Cessnas from the 70s are still running on their originally installed engine.
Defects and issues are still addressed ongoing, and maintaining the certification for things like modifications for improved power generation still means significant work. It’s definitely not a one-and-done thing - part of the value of certification is in the continuous future improvement that comes from NTSB findings or inspection reports sent to the FAA.
It’s like an engine fault light spread across the whole fleet. The certification keeps the light working.
Even if the design is certified, I’d imagine the shops that produce them would still have ongoing certification costs to validate the equipment, tooling, etc. On going costs that would have the distributed across a limited number of engines.
