Surely, like the film and music industries that we're so often compared to, there are a multitude of games each with a different market strategy. You have your EA sport sims, especially those around world cup 98 or euro 96, that we're marketed in the horizontal direction of every footie fan in England. This dragged in very soft core games who would buy anything with a football association. The same could be said of the latest barrage of Star Wars games. Mass marketed at a gaming arena not necessarily interested in games but certainly interested in the Episode One hype.
Then there's the basic genres of game, first person shooters, real time strategy's, flight sims, driving games, management games.
Games which, mostly, have no licenses and are there (hopefully) for the niche market's the genre definers such as C&C and wolfenstein created. These are more you vertical style of games, appealling to avid, perhaps hardcore, gamers, who love a particular style and buy the best of the genre.
Like films, you have your big budget players, who pump an inordinate amount of money into a single product knowing it will sell due to it's sequal value or it's having a license attached to it. There are smaller software houses who appeal to a niche market and have the occasional hit, allowing them to expand and appeal to a more horizontal group. As well as the dying breed of bedroom coders who write games that will only appeal to the hard core gamers due to it's lack of flashy graphics and shiny packaging.
I doubt you'll find that the percentages have changed over the last ten years of horizontal to vertical games produced, merely the size of the overall market that it appeals to. Being that there is a computer or console in the houses of almost everybody I know and a playstation comes a close second to cable tv here in England, the market is expanding with Moore's law. But so are developement costs and the entry level quality (denoted by look, not gameplay) of a game.
So in general, I think the games market has it's own brands of vertical and horizontally marketed games but the market itself is shifting towards a more horizontal percentage of the population.
Taking into account that we're talking in the first world...not developing country's...but I hear they're airlifting DreamCast in Ethiopia and Bosnia...so soon we'll have a truly global market.