Ah, that's where you and I differ wildly. Steam may have a lot of mind and market share, but that doesn't make any kind of enforceable monopoly on such an open platform. That's not even talking about going to other platforms.
Think of Apple's iTunes. There are still plenty of other places you can find and buy music. Customers with niche tastes get fed up with not having those tastes catered to and start looking outside the big services. That's why I'm here.
The vast, vast majority of Steam users don't look at Greenlight. Do you remember the early days of the service? The comments section of something like LS would have been
full of derision. "...Are these devs seriously trying to put a screensaver on Steam?" "lol looks like an app store game, stay on mobile", "Fucking indie non-games, GTFO".
Those people have stopped even caring. We're down to the people who are genuinely interested in new or niche titles here. So when the minority that cares enough to actually sift through Greenlight and vote still isn't interested in buying a particular game...the implication is that the portion of potential customers being excluded is anything but big.
I can sympathize with being part of a very small audience; I've given up hope on seeing the
Alltynex series Steam-side. At least Luxuria Superbia seems like the perfect type of game to become an IGF finalist and get on Steam that way. Pretty sure that's what happened with Bientôt lété.