If for example there was a true Instagram replica platform that forced adverts every 5 pictures or so, there is nothing stopping anyone from filtering it out, creating a new platform, creating an addon or whatever and give an identical view, sans adverts. The advertisers are essentially going to have to walk a line where rather than being able to force feed, they need to be ordered a la carte by the consumer, and each consumer will have the power because they are going to be able to access a single article from multiple points with multiple filtering options. That better be a damn good advert and not just a piece of crap from Wish.com

Force feeding ads is something that will not be as easy to do on decentralized platforms as on centralized ones. But I think you understimate the ease of creating alternative UIs without ads that can handle massive loads. Building a feature-rich UI with maximum usability is expensive and capacity to handle hundreds of millions of daily users is not free, either. I do think ads will be an essential feature on Steem UIs with mass appeal in the future.

Besides, advertisers and promoters have a way to go past the UI level. They can buy Steem Power and create content attractive to consumers, for example, contests that allow them to reward them. That is actually desirable as it will help maintain a high price of STEEM in a healthy, non-speculative way.

By the way, I wouldn't expect the most popular UIs to be open source for the very reason that it would be a threat to their monetizing ads. )The users' private keys can always be handled using SteemConnect.)