Eventually people lose interest. It's only human. But The System has been written, it is in production, and it has accrued a healthy stock of user data.
One day, The System breaks. "Only" tens of millions of users -- less than a percent of all Facebook users -- rely on the The System. But they rely on it utterly.
Who will fix The System?
Of the five engineers who wrote it:
* Jack and Mandeep left to launch myfornaxisnatrr.com
* Wei Li has moved to a different group
* John doesn't want to touch it, he only came on board to help Jack
* Michael was an intern but has since taken a job with Google.
Uh oh! There's nobody around to voluntarily fix an existing system. That's boring, and there are no incentives for fixing bugs in obscure features because only launching successful new features gains visibility from higher-ups.
Guess we'll need some mean old managers to round up a posse.
If they care enough.
Meanwhile, The System has acquired millions of users, cost millions of dollars to develop and operate and will now abruptly cause tens of millions of customers to become incredibly frustrated. And at no point has anybody stopped and asked:
"Was this the Right System to build?"