[alariccole]

This is discussed at length in the article. It was found that having an expiration lets people worry less about getting the photo "right". Also, it's not about having another place to waste time living vicariously. Quite the opposite, as it's becoming a vibrant community where you can actually interact in a surprising way.

That said, it is worth experimenting with the dates and/or ways of saving certain images of yourself for posterity.

[sp332]

That's an argument for an optional timeout, but it doesn't explain having a mandatory one.

[vbl]

Mandatory timeout means that even the best photos go away. No proliferation of the beautiful and perfect, no buildup of the quick and dirty.

Every one dies.

[alariccole]

True. I suppose the answer is that it was not the goal of the app to be in the photo storage/memories business. Looking at ways to help with this use case though, such as letting you save your own images to your camera roll.