This could even be a slow dark section after the finale that serves as an extended brake run. Disneyland has Rainbow Ridge, Paris has the tunnel beneath the river, and WDW has nothing. They could even just enclose the current stretch of track leading back to the station after the brakes and install two or three of the more famous Rainbow Caverns water features.
Do we know if the track replacement will necessarily be a massive, one-time undertaking? I wonder if they wanted to minimize downtime they could slightly extend Big Thunder’s annual refurbishments and replace a portion of the layout every year for three or four years until eventually the entire layout has been replaced.