> Communities would only make sense (from users point of view) if they would allow them to get more visibilty and exposure

They do. So you can think of it right now as you are a lone soldier fighting every other lone soldier for a spot on trending which more people will see. With Communities you now fight as a group to become a trending community. Or you can simply choose to post in a trending community for more visibility.

You can post in multiple communities by making multiple posts. Right now this is frowned upon and would clutter your blog, but in the future with the new interface you'll be able to choose which posts show up on your blog. So it wouldn't feel spammy to other users. I do hope that they add the functionality to resteem to another community though as it seems like that just makes more sense.

I think Communities costing 3 Steem does seem like a waste now, but I think it's future proofing. What about if Steem is worth $3 again? Then it costs $9 USD to make a community? That's a lot for a lot of the world already. If it were 10 or 100 Steem, sure, that's almost nothing right now, but this is crypto, and all this can change in the blink of an eye.

Not sure about your issue, I've been given the option to change roles in my community as the admin, no need to sign is as owner.

Communities cannot fade into irrelevance like tribes because they will fundamentally change the way Steem frontends work, it will change condenser which is what most of the other frontends are based on. Even if people don't go around making their own communities, they will still be using other peoples.

edit- Oh and one thing I forgot to address, it doesn't matter if we have a million empty communities as a community is basically just an account. We have tens of thousands of empty accounts already. They just won't be seen if they're not active.