Proton didn't exist yet, IIRC. The Steam Boxen relied on devs/studios/publishers being able and willing to port their games to Linux natively. The result was a handful of AAA and indie games that put proper effort into ports that ran well, a modest but larger selection of AAA games sloppily ported (such that they often can't run on current distros without containerization or extensive library preloading shenanigans), and a deluge of indie shovelware / forever-in-early-access vaporware produced by clicking the "gib me Linux" button in Unity and calling it a day.

Unfortunately, while it was certainly a boom in the number of games Linux users could play (easily enough for me to ditch Windows entirely and game exclusively on Linux and consoles), it wasn't quite the critical mass needed for Steam Boxen to be a commercial success. Proton was the missing piece.