Advertisement won't be something we "won't" allow, ads have become more evolved with time, a lot of things you consider not being ads nowadays might be ads without you knowing it. Even people are becoming part-time ads.
In a decentralized platform its going to be even harder to "shut down" or "not allow" ads, cause there will be fine lines between what is ads and what isn't.
The only thing I hope for now is that we won't add ads to the site (banners, side-ads, etc). I made a post about this 2 months ago: https://steemit.com/steem/@acidyo/ads and thinking about it now, I like it without ads a lot more and I'm sure many users won't even want to be paid in ads and sacrifice the clean feeling to the site. Also statistics like someone wrote earlier today prove that people don't want to see ads more and more (ad-block becoming more used). If they want to see their favorite bloggers, authors, etc use ads in their material, that's another thing.

I think in the future it will be a bit more like Instagram is today where companies and advertisers pay users with a lot of traffic for ads.