I'm very sorry for your loss...it's a rotten way to start the new year, but these things happen sooner or later- it's part of sharing our lives with animals.
I think you absolutely made the right decision not to wake him up to what could only have been suffering longer, & you should not blame yourself for waiting- sometimes it's very hard to know what's going on with reptiles, & he certainly fared better than if he'd been a wild dragon. Sad that he was only 5 years old though.
Very interesting what your vet said about his genetics & inbreeding being a possible cause, but that makes sense, I guess. Reinforces my preference for "wild types" as being healthier, though I'm sure there's plenty of exceptions.
A few years ago, I lost an older rosy boa to a nasty tail infection that appeared "out of nowhere" (no previous trauma, no mating etc). These things just happen sometimes.
I hope Nibbler has more time- what is his diagnosis? (abscess? cancer?) What is the plan with him?
Thanks for sharing Aegon's story (R.I.P. Aegon), I'm sure it helps for others here when they have to make similar & difficult decisions.