"A new gearbox is unlikely to fail, a newly installed recently overhauled gearbox is unlikely to fail."
not so. The majority of component failures come in the first 25% of component life since new or overhaul, followed by the last 25%. The middle 50% is the least likely to fail. That's why they run all components on the US presidential helicopter for 25% of its life on one of the other aircraft before installing it on the presidents aircraft. At 75% they pull it and scrap it. Sorry I don't have a source, (wish I did) but I've heard it several times and it aligns with what I have witnessed. A good friend of mine lost the C47 on his 407 last year with 80 hours TSN on the engine. #1 bearing was incorrectly pressed.