What if that legacy component was no longer in use? Is there some bigger picture in play?
I've been at bad companies before, so I understand being cynical. But, having people just do whatever they feel like also does not work. From the outside it looks like Google has quite a bit of this, so they are clearly trying to get people on some path. The messaging app situation shows it hasn't quite worked yet (from the outside anyway).
Said legacy component was (and is) making a substantial chunk of the companies revenue.
He was actually assigned by his manager to investigate the problem. Source: am a programmer.
If your manager asks you to do something, ask them how the thing they are assigning will help advance your career. Ask the manager how -- if you deliver on what they ask -- they will go to bat for you when they are sitting in the room with their peers justifying your evaluation score. Be willing and able to simply state, "I don't know how you can expect me to spend my time and energy on something that won't help me advance my career." If your manager can't understand or respect that, then that's great! You have a clear warning sign that it's time to fire your manager.