The point is if you want to report someone for it for sexual harassment, which is exactly what it is, then the disappearing message is a problem.
You don't just ignore it. You deal with it.
You don't just ignore it. You deal with it.
On the topic of unsolicited messages though, I prefer to tackle the problem at the source - it shouldn't be possible to message me at all if I haven't given you an invitation to do so, and invitations should be individually revocable. (This would be a big departure from the familiar phone number model, of course)
Technical reasons don't trump social and human reasons. There's a compromise somewhere.
The point is to spare the other party the hassle of having to think about their opsec after-the-fact, not to trap them in a setting they don't want.