It would effectively not do anything for game devs to reduce it by 5%.

On the dev side steam provides distribution and a bunch of tools while you develop your game. Tomorrow you can pay 100$, and steam will support you with keys, releasing and publishing your game, reviewing it for free etc.

I have a game I've been developing for 5 years part time. I have steam keys I share with testers, and can distribute version for free, with all the patch notes and update features from steam for 100$.

When I do release, they'll have earned the 30%, and if I don't release I'll have saved a ton and steam will take the costs. This greatly reduces the barrier to self-publishing. Out of all the companies I deal with, this is by far the fairest and lest predatory model there is. Gaben could have just bled us of our money even more and it would have worked. They are very rich because they are very humble in a sense.