The Star Trek franchise is one of CBS' most valuable catalog titles. It'll make back their investment, probably sooner rather than later. TNG in HD hasn't simply been made available on home video (a declining market), but is also being sold in television syndication (isn't it playing in the UK already?) and streaming/download services.
Sky has shown only 2 seasons in HD and there don't seem to be plans to show any more. Haven't heard of anywhere else putting it into syndication. And blu ray and dvd sales are still strong especially for fantasy/sci-fi shows. Look at how well Star Trek into Darkness did or the Hobbit or Games of Thrones. Even if the home video market is declining the portion of that market that TNG HD is able to utilise is remaining very small. Do we have any evidence that TNG HD is selling well any where at all? I'm the only person I know who has bought any of the remastered blu rays and I have many friends who are into Trek/sci-fi.