I think your framing is fair here. But I'd like to offer an even more complicated/nuanced take:

Designing in a group can be very difficult, and doing it well is a skill set that most people don't naturally have.

I think this explains your point 1. Why do people view designed docs as pointless? Because they really don't have a vision or model for what and effective and healthy collaborative design process would look like.

> Designing in a group can be very difficult, and doing it well is a skill set that most people don't naturally have

Not only is it a skill every participant needs to have, they also all need to have a similar amount of competence and knowledge about the domain as well as the current implementation, otherwise it's mostly pointless ime.

But if all ven diagram circles overlap ... It is nice. I can count the times this materialized (in my professional life) on one hand. So I'm generally more towards the "make a prototype, then explain it to the others. Either it's the baseline for the discussion or the illuminating event that clears up wherever this approach works with that team.