This isn't trying to be snarky, but I disagree almost entirely with what you wrote. Adding parks would absolutely help with alleviating demand. It might increase demand on the entire resort (let's just say WDW), but it wouldn't increase demand on any one individual park, or any single attraction. And while, yes, eventually demand can get to a point where supply is so limited as compared to demand that the value of service becomes so much less that demand stays static (we're there right now, btw - that's the point of this guy's video) ... but that doesn't mean more supply would necessarily bring more demand.
Put another way ... in Disney Resort experience, guest satisfaction will typically increase as the ratio of supply (things to do) is increased in comparison to demand (people at the resort wanting to do things). If you increase things to do (rides, eateries, shows) that can increase demand, but it will not necessarily cause so much more demand that the ratio of supply to demand decreases.
Put an even different way ... if you have 40 guests at WDW and 4 parks (as we do now), and you add an additional park, you'd have to add 11 more guests before supply/demand has gotten worse. That's a 25% increase in attendance. That's a very simple example. Now, let's say you have 10,000 guests at WDW and 100 attractions ... if you added one more attraction (without removing any), you'd have to see attendance go up by 101 guests before supply/demand is worse. That's a 1% increase in attendance. Obviously, these are rudimentary examples that don't factor in ride capacities, park hours, maintenance, staffing, optimal park capacities, etc - however, it does demonstrate the general principle that if you add things to do, guest experience will improve.
But it costs $. And right now $ is Disney's primary concern, not guest experience (and certainly not longterm guest goodwill).