I'm unfamiliar with SCOT's promotion system, be lets go with the the hypothetical situation that we as a community decided to keep vote selling in check because it's an indirect form of self voting and are mostly successful at doing so.

What are we going to do about circle-voters/daily content producers - like this?

Circle jerk groups are also an indirect form of pure self voting. A giving B $10 on a post in return for B giving A $10 on a post is the same as A self voting $10 and B self voting $10.

Therefore, they should be attacked by downvotes just as vote selling and self voting.

The justification for downvoting all these behaviors and others like it is the following: they're all forms of content indifferent voting behavior, usually (almost always) involving the stakeholder getting back the value of their vote in some form or another (sometimes minus fees etc.) at the expense of the vote reflecting one's subjective appraisal of content. In this sense they all harm the ecosystem as I described in detail in my previous comment.

So yes, I would absolutely counter these types of behavior with downvotes and wish to create a positive domino effect of honest voting on here. I can't get to everyone at once. It was mostly just my outrage at that particularly egregious offense on this post that I called you first to begin the conversation.

And what would be an alternative to these mechanisms, where people clearly want to profit with their stake? Only curation?

I believe if the economic incentives function correctly, then we can successfully strive for a culture that rewards behavior that at least do not undermine honest curation broadly across the platform. For example we have some part of the inflation devoted to rewarding staking returns in the form of direct interest. While I'm not a huge proponent of this and think it's fairly redundant, it can serve as additional motivation to stake and increase the security of the platform among other benefits. I don't believe it's damaging in the sense that 10-15m stake in your vote selling schemes is damaging because it doesn't undermine honest voting behavior.

should Steem even allow general content? Or should this be on an SMT?

This part is less directly relevant but I believe that our competitive advantage lies in a main token being able to reliably discovery and reward content based on subjective appraisal of content. I think pivoting to a general purpose token would be a strategic error as our currently rank 80 project is unlikely to be able to compete with the turing complete DPOS blockchains both technically and financially.

However, even if I thought differently here, the fact remains that Steem as it's currently designed is still purposed as a proof of brain token so my answers to your above two questions remain unchanged.

I don't believe it's a fair comparison to equate running large vote selling services now with self voting before the economic changes where fighting against the abuses I outlined was completely futile. I am not blaming you for running your services for the period during which I was self voting. Similarly, if I am currently still self voting, it would be most hypocritical to blame you for your vote selling bots.

However, the new economic incentives have paved a way now where we can potentially change the behavior of this entire place to a broad set of voting behavior that reflects honest appraisal of content. Your vote selling networks, which I'm guessing to be 10-15m SP strong in total is greatly undermining this effort.

You did express reluctance with running these bid bots once, which is why I'm certain you understand that effectively you're facilitating 10-15 million SP worth of self voters or circle jerkers and profiting from it. I'd like you to reconsider again please, as I expect top 20 witnesses to be more responsible.