People should give feedback to channel owners that overuse announcements. 

Send a DC. Tag them in cast.

Co-create the community you want it to be.

And if they are not receptive, leave / unfollow / create a new one.

Stop thinking of channels as canonical topics.

will channel owners ever have the option to make them public or curated channels only?

Public channel would be overrun with spam.

Your previous idea to split the feed into recent and power users (currently members) was good. Think about ways to implement this in the new version, maybe through filters or additional channel settings. For example, it may not be published content, but "suggested". 
The secondary garbage feed took the load off the main one, it was a bit weird, but it made sense.