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.
Me: That better be a damn good advert and not just a piece of crap from Wish.com
They are going to have to get much more clever to get access to the same sectors and, much more compelling.
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.
Yes, they are going to have to buy in, not commandeer and, they are going to likely have to profit share back to their users more. They are going to have to compete more directly for consumer attention.
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.
They are going to have to offer a lot and, they are going to have to keep increasing what they offer to the consumer.