Something in a financial services or similar large, bureaucratic, corporation where your job is to maintain an ancient system that everyone is afraid to change

Something in a financial services or similar large, bureaucratic, corporation where your job is to maintain an ancient system that everyone is afraid to change too much because all the greybeards that originally built it are gone but the company depends on it for some basic need. Note that this system will always be targeted for replacement by ambitious managers wanting to make a name for themselves but most projects will fail at the implementation stage if they get that far so you should be able to milk this cow for a fairly long time.