Businesses, and NBA teams are businesses, do not do things for the sake of doing things. If it isn't in the best interest of a team to make a trade, they won't. If Atlanta and Houston are not getting the return on Horford or Howard they think is fair, they shouldn't make a trade and can still decide what to do with the extra cap space this summer if they decide to leave. Both teams have many options, including, trying like hell to win with the players they have this year.
I said months ago that the Celtics wouldn't be players in this year's trade deadline transactions. Once NJ came out as poor as they did, the C's had to hold onto that pick and basically made them have little that others teams would want for a star player.