I have confidence a game published on steam will be there 20 years from now. I have much less confidence games on epic games store will be there 20 years from now. That's a huge value-add.
People keep putting Steam on their desktop because Valve has spent decades cultivating customer's trust, meanwhile every other MBA has been burning it for short term gain. Partnering with Valve comes with access to things like APIs and tools, but you also receive a little bit of that trust by association.
Not only has EGS seemingly failed to build something with equivalent features, APIs, tooling, etc. but in doing so they've failed to show a reason to trust them. So if the lower cut isn't resulting lower prices, what's the value prop for customers? GOG has an angles: games nobody cared to make available for a long time and being DRM free. To me it looks like Epic took the MBA route and wanted to build a moat via console style exclusivity, but that's not trust. Free games aren't trust. Just being on the customer's machine isn't trust.
(Also, these letters aren't current. iirc Vale doesn't simply take a flat 30% cut in the current day)
Steam has a track record of making their product better and better and the biggest scandal I can remember is when they introduced paid mods since that risked undermining the free mod market and GabeN did adjust the pricing afterwards and did personally get involved with the PR fallout. Otherwise the service has just gradually gotten better and better for the customer. Steam is privately owned instead of being owned by publicly traded companies and that helps with my confidence in the company.
Not by Steam.
However there have been many cases where developers pushed an update that removed content, which is really the same problem. You can't play GTA San Andreas or GTA 4 with the original soundtracks if you purchased them on Steam!
https://www.pcgamer.com/grand-theft-auto-san-andreas-steam-u...
https://www.polygon.com/2018/4/27/17292836/gta-4-soundtrack-...
This should be illegal.