For those saying that the receiver can always screenshot and share in #2, sure. But apps can lead the receiver in the mindset of the sender.
If you receive a simple link, without explanations, you will think it's ok to reshare. The link is public after all.
But if you receive the same link, with a little confidential note attached to it by the UI, you will try to follow your friend's wishes.
Remember, most DMs are between people that know each other and want to sustain a good relationship.
It's not about making it mathematically impossible to reshare.