People will purchase PS4 even without BC. BC is always overhyped but when you think about it's irrelevant for the most part in the end. What's the point of spending $400-500 on a new console to play 10-15 year old game PS1-2 games? People buy a new console to play next generation games. This is why they spend $400-500 as early adopters to have the latest and greatest in graphics and features on the console side. If there aren't enough next generation games to make a new console worth buying, they just wait 2-3 years for the library to build up and prices to drop and buy it then. All this time they can wait and play their PS1, 2, 3 games if they so desire. In contrast, BC means higher manufacturing costs, lower profit margins for Sony and/or higher costs to us consumers for having physical hardware that allows for proper BC to work. With Sony losing so much $, they cannot afford to incorporate the Cell CPU into PS4 and new components. It would also mean a much larger cooling system, case, motherboard, etc. Also, as consumers, I bet most of us would choose a cheaper PS4 than a $50-80 more expensive PS4 with BC.
For anyone who never purchased a PS2/PS3, they'd also be better off buying full HD/remastered remakes of the best PS2/3 titles at the Sony online store.