Steam entrenchement goes so much deeper than "just" network effect: DRM (and even DRM-less, to some extent, for convenience) requires trust and despite a choppy start, Steam has definitely built a lot of trust. Maybe even earned (debatable, but I tend to agree), but definitely built. Trust that they are likely to still be around n years down the line, and trust that they don't shove every imaginable profit maximization down their captive audience's throat.
They have a headstart of more than a decade to any upstart and the growing of trust cannot be artificially accelerated. On top of that there are financials. Valve are not expected to disappear any time soon because of their huge earnings. Again, impossible to replicate. And they are a private, mostly (or exclusively?) founder-owned corporation, so there is at least a possibility of making "good enough" money, which is important for the "not shoving unwanted features down customers' throats" part. A VC funded startup, or worse, a publicly traded company could never reach "good enough" profits. It's the goose that lays golden eggs: a publicly traded company would inevitably keep rising into overvaluation until it eventually reaches the point where the only way to justify it is to cut up the goose in search of even higher profitability.
