>Even if that’s true (and it gets repeated as a very general statement without general data to back it up, so I’m not sure), then why couldn’t payment processors either a) charge retailers in high-risk segments a higher transaction fee, b) charge merchants penalty fees if the number of chargebacks from their transactions exceeds some threshold, or c) compete with each other to serve higher-risk markets?

They do all of the above. There are obviously an endless number of niche payment providers who serve the segments. And they are... expensive and unreliable. Not suitable to a huge business like Valve that derives the majority of their revenue from mainstream products.

Found an explanation: the reputational risk they're talking about is 2022 pornhub case. Most likely some saudi dude didn't like it and threatened to pull. That's why they don't talk about it: because it's literal sharia exported around the world.

>That's the clause in the agreement they use to justify the increase in rates, but it's unclear it's actually "risky" when it's a large company like Valve.

The risk is not from Valve; disputes pass through to the processor. This is the same problem dating sites, gambling, etc. all deal with. Any sufficiently large adult oriented business becomes a de-facto payment processor, where their entire core business function is in managing this risk, not whatever they actually do for the end user. It's either that, or charge the exorbitant fees that come with using a niche provider who takes on that risk.