It doesn't work that way. If the offsite family uses days 6 and 7 to go to Universal, and the onsite family uses days 6 and 7 to stay at WDW, that doesn't mean that WDW has four fewer guests on days 6 and 7 (because the offsite family went elsewhere). Instead, the offsite family is simply supplanted by another offsite family. Stated another way, where does Disney reap the greater benefit...from an onsite family who buys 8 day passes, or from two off site families who each stay 4 days? Given the way
ticket prices work, Disney does better selling two different offsite families 4 day passes than it does selling one offsite family 8 day passes. The "they will go to other parks" argument only works if that results in a net decrease in attendance, which we know, it doesn't. Leaving hotel revenue out of the equation (because the offsite families don't contribute to that revenue stream), Disney would like nothing more than to have offsite families buy one day passes and then spend the rest of their vacation at other parks. Provided, of course, that they still get 100,000 offsite guests per day. As long as 100,000 offsite people turn the turnstiles, Disney has no interest in whether they stay for 3 days, 5 days, or 10 days. And it can easily be argued that Disney prefers the shorter stays. More ticket revenue, and a higher likelihood that the family stays in the park all day, eating all their meals and buying all their souvenirs on site.