What you get absolutely correct is that a perpetual reward system will be what wins in the long run, as we can all get paid for YouTube videos produced years ago. SteemIt differs, so this is a major concern for anyone who wants to use this platform. A great post after a period of time can't receive rewards - The End. This is where I would apply the promotions, not on a separate tag. Advertisers could swap their SBD tokens for promotions on popular posts, which would give the SBD tokens spent to the poster. So, as an example, if you wrote a great post about health and an advertiser wanted to sell a product related to your post, the advertiser would send SBD tokens to promote on your post and you would receive the SBD tokens for that post. This way content could still earn tokens provided that people want to advertise on those posts. If we think about it, this is fundamentally what's happening on YouTube and other social media platforms - someone is selling products on old content.

Also, because I think this is a good topic and am glad you highlighted it, I wrote One Possible Solution To SteemIt's Promotion Problem related to this and the promotion problem.

A Fix For This Now

Every post should have links to outside content, or else you are relying on this platform too much (and there's risk of its non-existence, like with YouTube or other social media).