It's pretty inevitable. The bottleneck on selling more games isn't really cost, it's player's free time.
For a while, there was a race to the bottom to tap into the expanding digital sales marketplace, hence all the back-catalogue stuff given away almost for free. However, that market has largely matured now, and people have huge backlogs of stuff. There are games you wouldn't even bother getting if someone said they were free: these games, to you, have negative value.
Say you're going to decide whether to offer a 50% off sale, or a 75% off sale. You are relying on their being twice as many sales at 75% off vs 50% off. If that's not the case, then you'd be a fool to do the 75% off sale. Most people willing to get your game "just because" at 75% off either already got it, or already go so many other damn games that they're "meh" about yours being 75% off.
Things that are worth the time will grow in value to encompass the market place. Some things have negative value (players don't see them as worth the time, even for free), but those games won't approach zero cost. They'll rise in price too, since some people will value them and be willing to pay, and you won't get additional sales from lowering the price, since most people won't have an interest in the game, even for nearly free.