On other hand how much developer time would have been spend on building own distribution, billing and related customer support. Time spend on doing it yourself would not be free either.

30% for this is high, but then there is also the discoverability. Which I think does beat google by long way. So they probably would not have sold as many copies without popular platform.

Nothing of what Steam provides costs 30%. Discoverability and free marketing only provided to games that are already successful and have hundreds of wishlists. That's only possible to achieve if you game already have it's own following and community.

12+ years ago if you released on Steam it was a big deal and platform provided traffic to everyone, but today it's flooded with games so basically you're on your own.

The only thing that allow Valve to charge this much is network effect. They are not vendor-locked platform like App Store, but they do have nearly monopoly on PC.

This is exactly what I was trying to point out. Steam's developer services can save a massive amount of time for developers, time which is especially valuable to indie studios. I still feel that the 30% is steep, I'd prefer if Steam took a cut based on how many of their underlying services you used, but its wrongheaded to deny that Steam provides many useful features for developers that can save a lot of time.

https://partner.steamgames.com/doc/features