I feel like Steam could remove their most favored nation clause (which is what this lawsuit is about) for any storefront that isn't selling Steam keys specifically, and the amount of sales they'd lose would be effectively a rounding error. I don't care if a game is 10% cheaper on EGS or itch.io or wherever else; I'm still buying it on Steam because I want to use the services Steam provides. The sole exception is GoG - but even with GoG, I still find it much less reliable than Steam for just being able to get the game working without problems (on linux specifically).
If the product being sold is a Steam key, I don't think there's any argument that could stand up against the MFN clause... the fact that Steam allows developers to generate Steam keys for their games for free and sell them elsewhere is pretty generous as it is now.