I mean, I don't remember the last time something came exclusively through Steam by exclusivity agreements. That cannot be said for Epic Games though.
Sure Epic had to come up with ways to increase their competitive advantage while having an inferior product with smaller catalog and worse ux, all problems inherent to a newer product trying to penetrate a consolidated market.
Steam felt like it was just there, adding features and improvements in a slow but steady pace and really offering value to both players, by introducing sales and democratizing access to games (sure in the EU and US it might seem less important, but coming from Brazil I can't begin to tell how much of an impact digital outlets had to providing access to quality games on emergent markets), and to developers by single handely making piracy somewhat obsolete.
Sure it had problems like high fees for small devs, a case can be said that this monopoly over time started hurting the small dev, or the massive flow of low quality games, but to call it a walled garden is a far cry from reality. We gamers got it good and we sometimes don't appreciate it. Just look at the state of streaming services to see what could have been.