That would work if the convergence was only from one side. You are assuming that (forgeting general price movement) that the spot stays still and the future differential decays away with time.
In practice, while they are both moving hundreds of pips, they do converge. So, of the intial 100 pips difference, 50 up (or down) will come from the spot and 50 down (or up) from the future cancelling the two trades out.
Aha, you say! Then why not do that and profit from the overnight swap (interest rate differential)? Because it won't be 50:50 and that's unpredictable.
However, if you find a pair with not much diverganence and a big interest differential (like the GBP/JYP) you can indeed make a profit from the swap - but it's tying up a huge sum of cash for a small gain. The theory is good and this is the carry trade but I'm not sure if I have the balls to risk the £10,000 it needs to make this worthwhile!