While I won't comment as to the validity of the lawsuit (that is for the courts):

1. The "standard" for selling video games involved needing a publisher who could coordinate with manufacturers and distributors to fight to get your big box onto a Best Buy shelf. Steam is one of the biggest "disruptors" in history. They don't get to make the "that is just how it always was" argument*
2. PC Game developers very much do not have more choices. Because, with very few exceptions, the response to "we are selling this on our own store" or "we are selling this as a gog/humble/epic exclusive" is "Fuck you, wake me up when you are on steam".

I don't know enough of the math behind the Steam CDNs and services to know if it is worth the cut. But, much like I am always going to whinge at DLC prices even as I acknowledge that it is "a good deal", I am also going to generally side with "devs deserve more money".

*: Take this with a grain of salt since it is a large claim and there are obviously no citations. But Steam did not invent digital distribution and companies like Strategy First (?) existed. And their cut for the massively inflated game prices (80 USD in the early 2000s...) was a LOT higher than 30%. Ironically, Valve used the same "you get more money if you sell with us" argument.