I am always thinking about Vinge's "A Deepness in the Sky" and computer archeology: there is an accumulation of the code base and at some point most code will be some kind of legacy.

It is not feasible to rewrite it in the fancy language/technology du jour - huge waste of resources for things which already work fine and at the moment you are done you have to start again, since the IT-pendulum is traveling in the opposite direction.

We, as an industry are really bad at working with legacy code, but for a lot of us it may be the main task if we endure this line of work for some decades more.