Adding capacity with new E-ticket attractions makes crowding worse, not better. You add a new attraction that increases capacity by 8,000 per day, and you end up with 12,000 new people showing up to ride it. And Disney fans think EVERYTHING is an E-ticket.
For a MK 8-10 operating day, that would mean the attraction can only handle 570 riders per hour. Disney has made some poor ride capacity decisions, but they aren't that bad. Working the other way, the MK needs a ride that serves somewhere between 2000-2500pph, and Universal has been able to build HP rides that fit the bill, so it's possible. Let's call it 2200, and a 14 hr operating day. That's about 30,000 new ride seats per day.
Looking at your other number, 12,000 new people a day would translate into an increase of 4.3 million guests per year. What is the historical precedent for something like that happening with a new Disney ride? I assume you weren't trying to be specific, but the potential benefit lies in the specifics. The delta between a solid capacity ride and realistic daily increase in guest count is meaningful. If the MK has 20 million guests, a 5% YTY increase is 1 million guests. Or an average daily increase of 2740 people. Significantly below the additional 30,000 daily ride slots. Assuming all the new people ride, that leaves 27,000 spots for the guests that are already there. 90% of the new attraction's capacity is to the benefit of people already in the park. So essentially half of MK daily guests get the opportunity to add +1 to their ride count for that day. Disney pegs that guest satisfaction metric at like 8-9 rides a day, right, (I know everyone on WDWMagic needs a far greater number), so a +1 is not nothing for how a guest feels about their day, especially if it's a really good experience (unlike the New Fantasyland stuff Mermaid / SDMT hassle which leaves people wanting so the +1 didn't feel like a +1). And a 5% increase is still probably too much of a bump in this day and age, so that would lead to more than half of guests getting the +1, and some guests don't care and don't need it, but all the shifting as people make different choices may lead to a better situation for some existing experience they do care about. But you need a *real* capacity increase, not just closing something else to avoid operational cost increases, because that brings down the delta.
The problem is if a ride costs half a billion dollars, and you're only going to bump guest counts by 1 million people a year or less, that's a long repayment schedule without making people pay for that new ride directly, like say, with some sort of line-skip option.