Making the combine mandatory certainly won't work. For one, the NBA only invites a certain number of players to the combine anyway, usually guys who are at least in the running for a first-round selection. Nobody wants to expand it to the hundreds of players that would have to be there if it were a requirement for being drafted. For another, just like the highest prospects take advantage of the system now, there's no reason to believe they wouldn't in the future. It's easy enough to get a doctor to say you have the stomach flu to get you off.
At least now the non-participation is honest - the top players don't need to out-compete everybody else because they've already done so, over and over. And the NBA remains a league where the players selected at the top of the draft don't bust out too often. Teams already have a pretty good idea of who the very top players are, and they don't really need to see them that one last time and get an official wingspan in order to be convinced to draft them. If ever non-participation in the combine becomes detrimental to the top players' draft stock, don't worry, they'll show back up.
I know we really like to get all of the official measurements on these guys for our own evaluations, but the NBA isn't running the Combine as a way to market to the draftniks. The draftniks get the Vegas Summer League, and it's working really well.