Not necessarily. As long as they adequately priced the season sets, they would be able to make enough money off of them. Also, if the companies completely stopped selling singles, then nobody would be able to buy them, and they would just end up turning to the more convenient season sets.
Since no company has actually tried doing this, before, it can't be said for sure whether it would work or not. But I believe that it would work. People will buy the singles because they are available, but with the economy still not improving, people may start being more cautious with their money, and decide that its not worth it to spend $25 on just 4 or 5 episodes of anime.