I don't think we've heard that last from Tiger or Phil as golfers. Quite frankly, I see Phil being a force to contend with for at least another 6-7 years. Meanwhile, Tiger will be around for at least another decade or however long it take him to break Jack's record. That's a whole different thread altogether.
Anyway, the state of the economy right now has a bigger impact on the time they spend designing courses. Furthermore, they both have young families and that takes a higher precedence for the both of them, especially now. However, when they both retire from competitive golf, I'm sure there interests will turn elsewhere. But I'm not sure if their efforts will be fully devoted towards golf course design.