I don’t really see the reason to credit here. As a product developer you obviously draw inspiration from your competition, and do whatever you need to keep up and hopefully get ahead.
This is in my opinion exactly the same as Instagram implementing stories from Snapchat. Doing so is perfectly fine, entirely legal, and in this sense being a gentleman and not implementing the feature will cause you to be set back compared to the competition.
Why are you bringing up the question of if it's illegal or not when no one else is? There's no dispute as to whether or not it's illegal -- it isn't. There's no argument there. There's realistically, nothing to be afraid of from a legal standpoint, which is what makes the behavior from organizations with large legal resources so puzzling.
People keep bringing up Facebook/Instagram and Snapchat. Facebook/Instagram has had no issue with admitting that Snapchat was an inspiration, so that doesn't help your argument. The issue here is credit and attribution, not legality or morality.
> The one thing you never hear in Silicon Valley is an entrepreneur admit they copied someone else. Yet there in the headquarters of Facebook, the world’s most prolific product cloner, Instagram CEO Kevin Systrom surprised me.
