The critical mass needed to tip the scales is not high. Once Linux has enough market share to matter as a customer base, game studios will switch to developing as cross-platform for it by default, so that they don’t lose launch sales. Once this happens, a lot of people won’t have any reason to stay on windows anymore as gaming was the only thing holding them back. This will then create a virtuous cycle of users migrating and games (and then apps) switching to it. Along then come hardware vendor supporty and then pre-built PCs and laptops. If the tipping point is reach, the rate of market share gain will be exponential.
The same thing happened with Internet Explorer 6
The only thing that can stop this is outside pressure from software giants like Microsoft through lobbying the Governments, buying out game studios or buying exclusivity, or strong-arming hardware vendors.