True. Also, to your point, one could argue that if that developer leaves, they'd have an easier time hiring anyone with Aurora experience as opposed to someone to learn and maintain the custom database.

But at the same time, Aurora costs could also scale with usage. It may cost $120k one year, $180k next year, $500k the year after. If the database they have now is well designed after it's already built it may not need active development every year but adding a feature here and there. Also, switching back to Aurora could also be an opportunity cost "we should have written our own thing and could have saved millions ...".

Well considering the cost is lower than Aurora isn’t the opportunity cost in favor of the home built situation.

That’s only true if Aurora is the most valuable thing that developer could be working on.

Agreed, a developer that can pull this off is pretty good, if maybe distracted by shiny objects, what could they do working on the actual product instead of this technological terror?