I think developers of a lot of big games and Microsoft initially realised that the game industry needed to move into a more 'Movie/TV industry position where they can have multiple avenues to gather revenue and this is where downloadable content comes in. If you can get $5-10 extra on average from your customers for the downloads of maps and extra content which is cheap and fits in with your development of the next game it makes perfect sense and it definately helps make the bigger projects even more profitable to offset the next game in development or other games which they aren't so lucky with. Movies have ticket sales, DVD sales, rentals, and tv broadcast. Games now have retail sales, DLC, games on demand (for cheap titles the cost of game + distribution/retail margins are higher relative to sale price) and hopefully in the future game rentals.
But as always good content prevails and companies like Valve, Epic, Bethesda are simply rolling in the cash because they simply make games which are standout in the quality and the sales follow through from that.
Tease.
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