wonder if this board is treated?? I bet it is not. It is a big task to remove the sliding door and the bottom track and repair the board underneath. you are talking about alot of labor dollars.
I would continue to protect the wood as you are. I would try to find some way to keep the water running off and away from this area. You can get some drip edge flashing and some how design a method to protect this area. maybe you could remove the bottom track screws and slide some 8 inch metal flashing under this bottom track and then make a downward 90 degree angle to protect this board.
Hard to say, but I know some day you will have to remove the complete door and fix this, when you do, use a treated wood product.