Steam

We can go back and look at the historical prices for The Division 2 and see that Ubisoft didn’t have a lower baseline price on their own store compared to the epic store. So either Epic has an MFN policy as well, or Ubisoft would most likely want to keep their prices consistent across platforms and stores.

Thanks for digging that up, interesting to note. Epic might have an MFN, or maybe Ubisoft's internal publishing overhead is roughly 12%.

That’s the thing: you’re being given a random game every week and that’s still not enough to get people to stick around

I don't know what you envision when you say "stick around". Do people uninstall Steam when they install Epic? No, they don't. You just have both installed. The free game gimmic is for you to download the platform; that's the first hurdle, but it does little to change your preference between platforms when it comes time to make a purchase.

And looking at the store now, it seems they’re just giving back 5% of the money you spend, meaning if you opt into their ecosystem, all their games actually are cheaper.

Interesting point on the 5%, I was unaware of that.

We also don’t really know that they do. The source saying that the MFN policy exists at all is the CEO of Epic Games saying so on twitter. And I’m pretty sure the lawsuit says that it’s “selectively enforced”, so there aren’t any actual examples of Valve vetoing a game’s price based on the price in another store.

What evidence would be needed to convince you?

Clearly, there is a business case for listing a game for less on Epic (or a publisher's own site!). We can trust the MFN policy most likely exists. What other explanation for the observed behavior can be put forth?

"Selectively enforced" is the wording used by Valve's own employee. That could mean anything from "only big, noteable games" to "only enforced when we noticed it" to "actually enforced consistently". Regardless, it can have a chilling effect that causes everyone to step in line.