I have a typical inswing double door (one is inactive) with two sidelights. I looks like it is sitting on a one long piece of threshhold - about 7.5 feet. There has been some leaks at the ends of the doors over time and I noticed that the aluminum sill on the concrete stoop is sinking down toward the door side, which may indicate that the wood underneath is rotting.

Is it possible to replace this threshhold with a new one, rather than replace the entire door? The doors are steel. Thanks for any advice.