it might elude you how ridiculously fucking expensive stuff gets

I don't know how that scales to worldwide CDN setup, but I work with a company who has presense on multiple countries and they drop casually 150-200k to few servers alone which provide services for at maximum for couple of thousand users. Networking, labor, power and things like that not included, just the hardware in a cardboard boxes.

Obviously there's a ton of factors on this and I can't elaborate our setup any further for obvious reasons, but just to give some scale how expensive things can get I can share my very real world experience. And that setup is pretty much the low end on the spectrum. 10k a month for a any as-a-service setup is almost a rounding error even with relatively low user count.

Serving anything to 100+ million people globally is a whole another beast. Wikipedia is a decent comparison, running a single website which looks like relatively simple to the end user (and oh boy it is not, but your Joe Average doesn't know nor care about that) takes around 170 million dollars per year just on operating costs.

I don't have any kind of opionion if that 30% is reasonable, but I do know that running that beast is not cheap and the money needs to come from somewhere. And as a customer, Steam just offers me what I want in a package which is the best one around, so they'll keep getting my pennies until something better comes along. And I'm very aware that their service is not perfect and that I don't really own anything on their platform, but for me in the current state in my life, they just provide the best bang for my buck and for the very limited time I have to spend with gaming.