I was with you up until this. I think anyone suggesting that a new attraction would reduce the demand and wait time for TSMM is crazy. If a new family friendly ride opened at Soundstage 1, it's not like people will say "I don't need to ride TSMM now" -- they'd want to ride the new ride and TSMM. And if the new ride actually draws a greater number of people to the park, those new people will want to ride TSMM. And I personally know of a number of folks who have been to DHS -- even with young kids -- and not ridden TSMM due to the long wait and not having FPs; if the ride had a sudden decrease in wait time then that crowd of folks would actually get in line to ride it.
Sure, if there were a lot of new rides added to DHS, then you might see a decrease in wait time/demand for TSMM. But we are talking at least 5 new rides, probably more like 10. As long as the number of rides in DHS remains in single digits and TSMM is one of only ones without a height restrictions, the vast majority of guests entering the gates are going to want to ride it. A single additional ride in SS1 is not going to dent demand for TSMM and will potentially make it worse.