we already have tags, which are extremely visible and accessible and intuitive

We do have tags - but Steemit could do a better job of driving people deeper into the site.

Many people might use it for years before they decide to join, and even then it's predicated by why they join, was it because of some members only area, did they want to leave a comment, etc.. To get people interested in powering up they must see good content,

On this we agree.

I simply believe:

• we are better off trying to get more people to the site - knowing only a percentage will power up
• once they are there, we can do a better job exposing content to them THEY value and are more likely to want to interact with and so join and power up
• acknowledge good content has a life of more than 7 days

Note my suggestions are focused on the content - not shifting the chairs around on the titanic by tweaking reward percentages or free downvotes.