Except adding new attractions generally increases demand and thus increases that amount of people. And while people will be excited to be able to ride Tron sometime next year, that won’t make them suddenly not want to ride pirates of the Caribbean. So the demand for Pirates of the Caribbean doesn’t really decrease.
Often conjectured, never proven.
In fact, it's already been dis-proven in this very thread with evidence:
Just to give an idea, here is the data from 2011-2019. Infer what you will about how additions have affected total attendance at the parks individually and combined.
View attachment 685359
The only place where there was a notable attendance bump over the previous decade was with Avatar in Animal Kingdom* and when you look at the impact of all four that year, it was considerably less pronounced, meaning, it didn't bring that many
extra people in.
NFL?
Apparently, had no real impact.
TSL?
Same.
The problem was, the additions weren't
enough to noticeably reduce crowding for most guests - a matter of too little, too late not helped by apparently 2/3 of their capacity being fed into FP+/G+ so the majority of riders for those areas just had another scheduled wait time and nothing to do but wander around the park and clog up other lines before doing their 3-5 minute attraction and being spit back out - much like they were already doing.
In the case of Avatar, the people already visiting the resort were just more likely to pick Animal Kingdom on a given day over one of the other three parks which is exactly the kind of effect some (most?) of us want.
I'm sure the bump for Star Wars was probably higher
(not at all helped by the delay and then still ongoing unreliability of the marque attraction) but that isn't because they added something new. It's because of the insane popularity of that particular IP.
I'd expect that M&MRR and RRA combined wouldn't have even a quarter of the impact.
Now just imagine if they were opening new things
regularly. The impact would be even less because the same "family from (pick your state)" isn't going to suddenly stop saving up to visit and instead mortgage their home to quadruple the number of trips they take over a ten year period.
It would thin crowds but it also would cost money and wouldn't provide much benefit to Disney management. They make more money when they're squeezing more people in with less to build, operate, and maintain -
especially now that they're monetizing the waits with G+ and ILL.
The only thing that I think is going to change that is guest satisfaction dipping low enough to hurt their margins and then they'll feel like the have no choice but to respond.
*Also interesting was that the same person who gave us that chart pointed out in another post that what Avatar did appear to bring into AK in total, was less than the daily capacity of the two new attractions, meaning it really was a net win for overall capacity and crowding even in just that park.
All the people choosing to wait multiple hours to ride them made that intuitively difficult to observe if you were someone there also wanting to do the only new stuff in all of WDW at the time, though.