If you look at the list of games with Linux support at Steam you will notice that virtually all of them has Mac support, which supports your idea about Linux and Mac support.
Most developers use 3rd Party Engines anyway, so they are also limited by the fact that many game engines dont support Linux/Mac yet. But that number is definately growing!
Also porting old games is one thing, but these days most 3rd party game engines support Linux, too, and it should be even easier to support Linux at launch with new games.
Considering Linux users buy about the same amount of games (in revenue) as Mac users right now (from the surveys/research I've seen), it should be a no-brainer to support Linux if you're already going to support Mac. I could see some not wanting to bother with Mac either, though, but for those who choose Mac, they should also choose Linux.