What's interesting is that Slack (the company) was founded in 2009 and yet the initial release of Slack was in 2013. That's 4 years of development, that seems like quite a long time.
Slack: They first built a game, it didn't do well, so they pivoted to focus on the message system they built for the game. Flickr: They first built a game, it didn't do well, so they pivoted to focus on the image hosting they built for the game. Games are hard.
I was sure there were more examples, though.
Games are hard.