I replaced the threshold on the exterior door on my garage. Used that same threshold but purchased it at Menards, it's an exact replacement for the mastercraft doors that Menards sells. My prehung steel door and wood frame was still in good shape, except that the threshold had been loose which had allowed water to get in. That rotted off the very bottom of the frame, mostly just where the original threshold had originally been stapled to the frame. The door had an aluminum threshold with some soft wood formed to the underside of it and the wood door frame was stapled to that. Most builder grade prehung doors that you get at the home centers aren't really made of the best materials. The prehung frame was just a primed, soft wood.
Rather than replacing the bottom of the frame, I used an oscillating tool and cut the rotted frame off right at the top of the threshold. I attempted to kind of match the new threshold profile. Then cut the new threshold so it was wide enough to fit well past the door frame almost to the studs. I removed the adjustable sill portion and cut it off both ends, so it would fit only between the door frame sides. Used Vulkem sealer under the threshold and drilled in some concrete screws in the space under the adjustable sill. Once I reinstalled the adjustable sill you can't see the screws. I also added a piece of composite lumber to the edge of the slab to better support the outer lip of the new threshold. The outer edge being unsupported is often the reason the sills get loose. People step on the unsupported edge, bend it down and breaks it all loose.
Once everything was back in place and tight I caulked the gap between the frame and the threshold with some Vulkem, polyeurethane caulk. Been about six years and it's still solid and working fine. The caulking still looks good and has not leaked any water in. Six years and should get several more for a fraction of the cost or work of replacing the whole door.
