I say this despite the fact that stories was what really brought stratospheric growth to snapchat, because i'm taking into account opportunity cost.
They could have focused on making their core (1-1 messaging) better but they decided to become a broadcast media company, starting with stories and then those brand videos.
This has always been risky. Even at its peak when you look at what was happening on Snapchat stories, a lot of these microcelebrities on snapchat would use stories as a way to get more likes or follows on their instagram. They say stuff like "Hey i just posted a new photo on instagram, please like it!" etc. This was convenient for them because these needy behaviors go away on snapchat stories.
The moment snapchat decided to become 1:many, they became a serious threat to Facebook in pretty obvious ways, so it was no brainer that Facebook jumped to do something about it. And at the same time, you can't compete with Facebook on 1:many broadcast.
They should have focused on 1:1 and fought on their own turf. Sure it would have been harder to figure out the viral growth, but it was already doing well so I'm sure they could have figured out some way without jumping into Facebook's playground.