But I thought Disney didn't have any slow times anymore?
Not this again. It's about capacity *and* utilization.
Take Mickey. The ride will have a higher poh throughout than GMR. Plus it will likely be more popular (I've been on GMR many times with empty seats). As much as I love GMR, that's a net gain, although not as much as if they had built it net new.
Take Star Wars. It is replacing Backlot tour and LMA. Two attractions that were also very underutilized. So again, probably a net gain there too. Part of proper park planning is building new, of course, but also seeing what isn't drawing crowds and helping with the overall capacity, and either fixing or replacing those things so they do draw.